Chromium-containing steel sheet for current collector of nonaqueous electrolyte secondary battery

Abstract

This invention provides a chromium-containing steel plate for the current collector of a non-aqueous electrolyte secondary battery. It exhibits excellent corrosion resistance in battery environments and, when used as a current collector, demonstrates excellent rate performance and cycle characteristics of the non-aqueous electrolyte secondary battery. The chromium-containing steel plate for the current collector of a non-aqueous electrolyte secondary battery has a composition containing 10% by mass or more Cr, a particle size Sa specified in ISO 25178 of 0.15 μm to 0.50 μm, and a particle size Ssk greater than 0 specified in ISO 25178.

Description

Technical Field

[0001] This invention relates to chromium-containing steel plates for current collectors of non-aqueous electrolyte secondary batteries, and more particularly to chromium-containing steel plates for current collectors of non-aqueous electrolyte lithium-ion secondary batteries. Background Technology

[0002] In recent years, from the perspective of protecting the Earth's environment, the production of electric vehicles (EVs) equipped with lithium-ion secondary batteries (LIBs) has been increasing.

[0003] In the current collector of a liquid metal arc furnace (LIB), Al foil is mainly used as the positive electrode and Cu foil as the negative electrode. To improve durability, the application of stainless steel foil, which has higher strength and higher corrosion resistance, in LIB current collectors has also been studied.

[0004] For example, Patent Document 1 discloses a bipolar battery in which stainless steel containing 16-26% by mass of Cr and 0.5-7% by mass of Mo is used as the current collector foil, so that even if it is used for a long time at a high potential (about 4.2V), the current collector foil will not corrode and dissolve, and the battery characteristics will not easily degrade.

[0005] In addition, Patent Document 2 discloses a ferritic stainless steel for battery structural components, which contains Cr: 16.0 to 32.0%, C: less than 0.015%, Si: less than 0.5%, Mn: less than 2.0%, and the surface composition ratio is Cr / (Cr+Fe) > 0.2, and has a passivation film of less than 4 nm, so that the battery can maintain high corrosion resistance and is not prone to sparks even when the voltage is high.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2007-242424

[0009] Patent Document 2: Japanese Patent Application Publication No. 2009-167486

[0010] Patent Document 3: Japanese Patent Application Publication No. 2010-33782 Summary of the Invention

[0011] However, the results of actually applying the stainless steel described in Patent Documents 1 and 2 to the current collector of lithium-ion secondary batteries and evaluating the battery performance show that, compared with the use of Al foil or Cu foil in the current collector, the discharge rate is sometimes higher or the battery capacity is sometimes more prone to decrease during repeated charge and discharge.

[0012] The reasons for the deterioration of battery capacity characteristics (hereinafter referred to as rate characteristics) at high discharge rates and during repeated charge-discharge cycles (hereinafter referred to as cycle characteristics) are not clear, but the inventors have considered the following: Al foil and Cu foil, which are mainly used in the current collectors of lithium-ion secondary batteries, have high surface conductivity. On the other hand, stainless steel plates have lower conductivity compared to Al foil and Cu foil surfaces due to the formation of a stable passivation film. Therefore, it can be considered that the interface resistance (hereinafter referred to as interface resistance) between the current collector and the battery active material layer formed thereon is more likely to be higher than that between Al foil and Cu foil, resulting in deteriorated rate characteristics and cycle characteristics. Furthermore, it can be considered that repeated charge-discharge cycles will cause a high-resistivity film to form on the surface of the stainless steel plate, increasing the interface resistance and further deteriorating the cycle characteristics.

[0013] Furthermore, for steel plates used in the current collectors of non-aqueous electrolyte lithium-ion secondary batteries, corrosion resistance in the battery environment is required. That is, in the environment of non-aqueous electrolyte lithium-ion secondary batteries, the steel plate must not cause a decline in battery performance due to corrosion.

[0014] In steel plates used for current collectors in non-aqueous electrolyte lithium-ion secondary batteries, if the corrosion resistance is low in the battery environment, corrosion products or pitting will form on the surface of the current collector. The inventors believe that this hinders electron migration between the current collector surface and the active material, thus degrading battery performance.

[0015] The current collector of a non-aqueous electrolyte lithium-ion secondary battery is exposed to an environment in which a non-aqueous electrolyte containing Li salts such as LiPF6, LiBF4, and LiClO4 dissolved in organic solvents such as dimethyl carbonate, diethyl carbonate, ethylene carbonate, and ethyl methyl carbonate is applied, and a voltage of 0.0–5.0 V (vs. Li / Li) is applied. + A special environment within the potential range of ) (hereinafter referred to as the battery environment).

[0016] The inventors believe that, in the aforementioned battery environment, by setting the Cr content of the chromium-containing steel plate to a certain value or above, the Cr oxide film formed on the surface of the steel plate becomes stable, thereby ensuring corrosion resistance in the aforementioned battery environment.

[0017] The present invention was made in view of the above circumstances, and its object is to provide a chromium-containing steel plate for the current collector of a non-aqueous electrolyte secondary battery, which has excellent corrosion resistance in the battery environment, and when used as the current collector of a non-aqueous electrolyte secondary battery, the non-aqueous electrolyte secondary battery has excellent rate performance and cycle performance.

[0018] The inventors have conducted in-depth research to solve the above-mentioned problems.

[0019] As described above, the inventors believe that the deterioration of the rate performance and cycle performance of the battery is due to the increased interfacial resistance between the current collector and the battery active material layer formed thereon.

[0020] Patent Document 3 discloses a current collector for lithium-ion secondary batteries, comprising a ferritic stainless steel plate with a mass percentage composition of Cr: 12-32%, C: 0.015% or less, Si: 0.5% or less, Mn: 2% or less, and the remainder being Fe and unavoidable impurities, and having a roughened surface with a surface roughness SPa of 0.1 μm or more. Furthermore, Patent Document 3 describes a technique that, by roughening the surface of the stainless steel plate and allowing a portion of the active material layer to enter the pits of the roughened surface, improves the adhesion between the current collector and the active material layer, increases the contact area, and significantly reduces the contact resistance between the stainless steel plate and the active material layer. However, the inventors have conducted research on the roughening of the steel plate surface and found that simply achieving a surface roughness SPa of 0.1 μm or more is insufficient for improving the rate performance and cycle characteristics of the battery.

[0021] Therefore, in order to suppress the increase in interfacial resistance, the inventors carefully studied the influence of the surface shape of the chromium-containing steel plate on battery characteristics. Their results showed that if specific surface shape parameters could be appropriately controlled, the rate performance and cycle performance of the battery could be improved.

[0022] Specifically, the conclusion is that in order to improve rate performance and cycle performance, an uneven structure is formed on the surface of the chromium-containing steel plate, and the protrusions are made sharper so that they penetrate the electrode active material. In this way, by increasing the contact area between the steel plate surface and the electrode active material, the interfacial resistance can be reduced.

[0023] Based on this concept, the inventors further studied and discovered the following insights: by forming a specified uneven structure on the surface of a chromium-containing steel plate, even when using a chromium-containing steel plate as the current collector in a non-aqueous electrolyte secondary battery, the rate performance and cycle performance can be improved.

[0024] Furthermore, the chromium-containing steel sheet (the chromium-containing steel sheet in the billet) is subjected to an immersion treatment using an acidic aqueous solution containing hydrogen peroxide, copper ions, and halide ions, with the treatment temperature and treatment time set to 30–50°C and 40–90 seconds, respectively, as the first immersion treatment. After the first immersion treatment, the following further processes are performed: (A) using an acidic aqueous solution containing hydrogen peroxide, with the treatment temperature and treatment time set to 30–60°C and 5–120 seconds, respectively; (B) using an aqueous solution containing nitric acid, with the treatment temperature and treatment time set to 30–60°C and 5–120 seconds, respectively; or (C) a combination of the above (A) and (B), as the second immersion treatment. This process forms a specified uneven structure, which improves the rate performance and cycle performance of the battery.

[0025] Furthermore, by performing a second impregnation treatment using an aqueous solution containing nitric acid, and controlling the ratio of the atomic concentrations of Cr and Fe in non-metallic forms to the total atomic concentrations of Cr and Fe in metallic forms [non-metallic forms (Cr+Fe)] / [metallic forms (Cr+Fe)] to 8.0 or less on the surface of the chromium-containing steel plate, a further reduction in interfacial resistance can be obtained.

[0026] That is, after the first immersion treatment (etching treatment), contaminants (a mixture of C, N, S, O, Fe, Cr, Ni, and Cu as the main constituent elements with high resistance) are generated on the surface of the steel plate. If these contaminants remain, even if the desired uneven structure is obtained, they may still contribute to an increase in interfacial resistance. Based on this, by performing the second immersion treatment (contaminant removal treatment) after the above etching treatment, the contaminants are removed, thereby stably achieving a reduction in interfacial resistance. In particular, by using an aqueous solution containing nitric acid as the treatment solution for the second immersion treatment, a further reduction in interfacial resistance can be obtained.

[0027] The inventors considered that the amount of contamination on the steel plate surface is related to the ratio of [non-metallic (Cr+Fe)] to [metallic (Cr+Fe)]. It can be said that the lower the ratio of [non-metallic (Cr+Fe)] to [metallic (Cr+Fe)], the more thoroughly the contamination is removed. Therefore, by controlling the ratio of [non-metallic (Cr+Fe)] to below 8.0, the interfacial resistance is further reduced, and even when using chromium-containing steel plates in the current collector of non-aqueous electrolyte secondary batteries, the rate performance and cycle performance can be further improved.

[0028] This invention was completed based on the above insights and through further repeated research. Specifically, the essential elements of this invention are as follows.

[0029] [1] A chromium-containing steel plate for the current collector of a non-aqueous electrolyte secondary battery, having a composition containing more than 10% by mass of Cr, and having a parameter Sa of 0.15 μm to 0.50 μm as specified in ISO 25178, and a parameter Ssk greater than 0 as specified in ISO 25178.

[0030] [2] The chromium-containing steel plate used for the current collector of the non-aqueous electrolyte secondary battery according to [1], wherein the ratio of [non-metallic form (Cr+Fe)] to [metallic form (Cr+Fe)] is 8.0 or less.

[0031] Here, [non-metallic form (Cr+Fe)] / [metallic form (Cr+Fe)] refers to the ratio of the total amount of Cr and Fe existing in non-metallic forms to the total amount of Cr and Fe existing in metallic forms on the surface of the aforementioned chromium-containing steel plate.

[0032] According to the present invention, a chromium-containing steel sheet is obtained that exhibits excellent corrosion resistance in battery environments and, when used as a current collector in a non-aqueous electrolyte secondary battery, excellent rate performance and cycle characteristics.

[0033] By using the chromium-containing steel plate of the present invention in the current collector of non-aqueous electrolyte secondary batteries, the rate performance and cycle characteristics of the battery can be improved. The chromium-containing steel plate of the present invention is particularly suitable for use in the current collector of non-aqueous electrolyte lithium-ion secondary batteries. Attached Figure Description

[0034] Figure 1 This is a schematic diagram (cross-sectional view) illustrating the contact situation when a chromium-containing steel plate (current collector) and an electrode active material come into contact according to one embodiment of the present invention.

[0035] Figure 2 This is a schematic diagram (cross-sectional view) showing the surface state when Ssk (skort) is positive (greater than 0) and negative (less than 0). Detailed Implementation

[0036] (1) Chromium-containing steel plate for current collector of non-aqueous electrolyte secondary battery

[0037] The following describes a chromium-containing steel plate for the current collector of a non-aqueous electrolyte secondary battery according to one embodiment of the present invention. In the chromium-containing steel plate for the current collector of the non-aqueous electrolyte secondary battery according to one embodiment of the present invention, it is important to provide a defined uneven structure on the surface of the steel plate. Due to the aforementioned uneven structure, such as... Figure 1The description states that the sharp, pointed protrusions (peaks) on the surface of the chromium-containing steel plate penetrate into the electrode active material. Furthermore, by increasing the contact area between the steel plate surface and the electrode active material, a reduction in the interfacial resistance between the current collector and the battery active material layer formed thereon is expected. This reduction in interfacial resistance improves the rate performance and cycle life of the battery. It should be noted that in this specification, the chromium-containing steel plate used for the current collector of the non-aqueous electrolyte secondary battery of the present invention is simply referred to as a chromium-containing steel plate.

[0038] [Parameter Sa specified in ISO 25178: 0.15 μm to 0.50 μm]

[0039] As mentioned above, it is important to provide a specified uneven surface texture on the chromium-containing steel sheet. From the viewpoint of obtaining excellent rate capability and cycle performance, this specified uneven surface texture requires the parameter Sa, as specified in ISO 25178, to be set to 0.15 μm to 0.50 μm. Here, Sa is a surface roughness parameter specified in ISO 25178, representing the arithmetic mean height. The arithmetic mean height refers to the average of the absolute values ​​of the height differences between points on the average surface, and is a commonly used parameter for evaluating surface roughness.

[0040] Here, when Sa is less than 0.15 μm, the height of the protrusions (peaks) is too low, preventing the protrusions on the surface of the chromium-containing steel plate from fully penetrating the electrode active material, or the contact area with the electrode active material cannot be sufficiently increased, resulting in no reduction in interfacial resistance. Consequently, rate performance and cycle performance decrease. On the other hand, if Sa is greater than 0.50 μm, the amount of dissolution and etching time increase during the etching process, which is detrimental to production. Furthermore, if etching is performed with Sa greater than 0.50 μm, the thickness of the current collector becomes uneven, resulting in localized etching, thinning of the current collector, and potential damage. Therefore, Sa is set to 0.15 μm to 0.50 μm. Sa is preferably 0.20 μm or more.

[0041] [The parameter Ssk (skewness) specified in ISO 25178: greater than 0]

[0042] Ssk is a surface roughness parameter specified in ISO 25178, representing the symmetry of the height distribution. For example... Figure 2As shown, when Ssk (skexibility) is positive (greater than 0), the tip (upper end) of the convex part (peak) becomes sharper, and the tip (lower end) of the concave part (valley) becomes wider (approaching flat). On the other hand, when Ssk is negative (less than 0), the tip of the convex part (peak) becomes wider (approaching flat), and the tip (lower end) of the concave part (valley) becomes sharper. Therefore, if Ssk is greater than 0, the tip of the convex part (peak) on the steel plate surface becomes sharper, and the sharp tip of the convex part on the steel plate surface easily penetrates the electrode active material, thus reducing the interfacial resistance. On the other hand, if Ssk is less than 0, since the tip of the convex part (peak) on the steel plate surface is flat, the electrode active material is not easily penetrated by the convex part on the steel plate surface, and the interfacial resistance is difficult to reduce. Preferably, Ssk is 0.10 or higher. There is no particular upper limit to Ssk, but by suppressing excessive increase of Ssk, it is easy to suppress the reduction of the number of bumps. From the perspective of easily and more stably obtaining the effect of reducing interface resistance, Ssk is preferably 1.00 or less, and more preferably 0.50 or less.

[0043] It should be noted that Sa and Ssk can be measured according to ISO 25178. For example, a laser microscope can be used as the measuring apparatus. Furthermore, in a chromium-containing steel plate for the current collector of a non-aqueous electrolyte secondary battery according to one embodiment of the present invention, Sa is set to 0.15 μm to 0.50 μm on at least one side (the side in contact with the electrode active material), and Ssk is set to be greater than 0. Chromium-containing steel plates manufactured through the etching process described later generally have Sa set to 0.15 μm to 0.50 μm on both sides, and Ssk is greater than 0.

[0044] [Composition of chromium-containing steel plate]

[0045] The chromium-containing steel sheet of the present invention has a composition containing 10% by mass or more of Cr. Cr has the effect of improving corrosion resistance in battery environments. By setting the Cr content to 10% by mass or more, corrosion resistance in battery environments can be ensured even when used as a current collector in non-aqueous electrolyte secondary batteries, particularly for current collectors in non-aqueous electrolyte lithium-ion secondary batteries. In the chromium-containing steel sheet of the present invention, there are no particular limitations on the components other than Cr, but the following composition can be cited as a preferred composition.

[0046] [Preferred Composition of Chromium-Containing Steel Sheet]

[0047] The chromium-containing steel plate of the present invention preferably comprises, by mass%, C: 0.001–0.100%, Si: 0.01–2.00%, Mn: 0.01–2.00%, P: less than 0.050%, S: less than 0.010%, Cr: 10.00–32.00%, Ni: 0.01–30.00%, Al: 0.001–0.500%, and N: less than 0.100%, with the remainder being Fe and unavoidable impurities. The above composition may further include, as needed, one or more of the following by mass%, Mo: 0.01–2.50%, Cu: 0.01–0.80%, Ti: 0.01–0.45%, Nb: 0.01–0.60%, and V: 0.01–0.30%.

[0048] The reasons are explained below. It should be noted that, unless otherwise stated, the "%" in the composition refers to the percentage by mass.

[0049] C: 0.001~0.100%

[0050] C reacts with Cr in the steel to form Cr carbides at the grain boundaries, resulting in a Cr-deficient layer and thus reduced corrosion resistance. Therefore, from a corrosion resistance perspective, lower C content is better, preferably 0.100% or less. More preferably, the C content is 0.030% or less, and even more preferably 0.020% or less. It should be noted that there is no particular limitation on the lower limit, but the lower limit for C content is preferably 0.001%.

[0051] Si: 0.01~2.00%

[0052] Si is an effective deoxidizing element and is added during the steel smelting stage. This effect is achieved when the Si content is 0.01% or more. Therefore, the Si content is preferably 0.01% or more. However, if the Si content is excessive, the steel hardens and its manufacturability decreases. Therefore, the Si content is preferably 2.00% or less. The Si content is more preferably 1.00% or less, and even more preferably 0.60% or less.

[0053] Mn: 0.01~2.00%

[0054] Mn is an effective deoxidizing element and is added during the steel smelting stage. This effect can be achieved when the content is 0.01% or more. Therefore, the Mn content is preferably 0.01% or more. However, if the Mn content is greater than 2.00%, the corrosion resistance tends to decrease. Therefore, the Mn content is preferably 2.00% or less. The Mn content is more preferably 0.60% or less.

[0055] P: below 0.050%

[0056] Since phosphorus (P) reduces ductility, a low P content is preferred. However, if the P content is below 0.050%, a significant decrease in ductility will not occur. Therefore, the P content is preferably below 0.050%. A P content is more preferably below 0.040%. There is no particular limitation on the lower limit, but excessive P removal will increase manufacturing costs. Therefore, the P content is preferably above 0.010%.

[0057] S: below 0.010%

[0058] Sulfur (S) combines with manganese (Mn) to form MnS, which becomes the starting point for corrosion and thus reduces corrosion resistance. However, if the S content is below 0.010%, a significant decrease in corrosion resistance does not occur. Therefore, the S content is preferably below 0.010%. There is no particular limitation on the lower limit, but excessive desulfurization will increase manufacturing costs. Therefore, the S content is preferably above 0.001%.

[0059] Cr: 10.00~32.00%

[0060] By containing 10% or more Cr, corrosion resistance in battery environments can be ensured, especially in current collectors for non-aqueous electrolyte secondary batteries, particularly non-aqueous electrolyte lithium-ion secondary batteries. Therefore, the Cr content is 10% or more. The Cr content is preferably 10.00% or more, more preferably 16.00% or more. On the other hand, if the Cr content is greater than 32.00%, the precipitation of the σ phase may reduce toughness. Therefore, the Cr content is preferably 32.00% or less. The Cr content is more preferably 25.00% or less.

[0061] Ni: 0.01~30.00%

[0062] Ni is an element that effectively improves the corrosion resistance of stainless steel. Furthermore, Ni is generally present in a certain amount in austenitic stainless steel and ferritic-austenitic duplex stainless steel. However, if the Ni content exceeds 30.00%, hot workability decreases. Therefore, when Ni is present, the Ni content is set to 30.00% or less. More preferably, the Ni content is 20.00% or less. Additionally, the Ni content is preferably 0.01% or more.

[0063] It should be noted that the preferred lower limit for Ni content in austenitic stainless steel and ferritic-austenitic duplex stainless steel is 2.00%.

[0064] Furthermore, when ferritic stainless steel contains Ni, the Ni content is preferably 2.00% or less, and more preferably 1.00% or less. It should be noted that the preferred lower limit for the Ni content in ferritic stainless steel is 0.01%.

[0065] Al: 0.001~0.500%

[0066] Al is an element used for deoxidation. This effect is achieved when the content is 0.001% or more. Therefore, the Al content is preferably 0.001% or more. However, if the Al content is greater than 0.500%, the ductility decreases. Therefore, the Al content is preferably 0.500% or less. The Al content is more preferably 0.150% or less, and even more preferably 0.100% or less.

[0067] N: below 0.100%

[0068] If the nitrogen content is greater than 0.100%, the ductility decreases. Therefore, the nitrogen content is preferably 0.100% or less. More preferably, it is 0.030% or less. There is no particular limitation on the lower limit, but excessive nitrogen removal leads to increased costs. Therefore, the nitrogen content is preferably 0.002% or more.

[0069] The above are preferred as basic components, but the following components may also be included in this invention.

[0070] Mo: 0.01–2.50%

[0071] Mo is an effective element for improving corrosion resistance. This effect is preferably achieved with a Mo content of 0.01% or more. However, if the Mo content exceeds 2.50%, it leads to steel embrittlement. Therefore, when Mo is present, it is preferable to set the Mo content to 0.01% to 2.50%.

[0072] Cu: 0.01–0.80%

[0073] Cu is an effective element for improving corrosion resistance. This effect is preferably achieved with a Cu content of 0.01% or more. However, if the Cu content exceeds 0.80%, hot workability decreases, leading to reduced productivity. Therefore, when Cu is present, it is preferable to set the Cu content to 0.01% to 0.80%.

[0074] Ti: 0.01~0.45%

[0075] Ti, by combining with C and N, prevents excessive precipitation of Cr carbonitrides in steel, thus inhibiting a decrease in corrosion resistance (sharpening). These effects are achieved when the Ti content is 0.01% or higher. On the other hand, if the Ti content is greater than 0.45%, workability decreases. Therefore, in the case of Ti content, it is preferable to set the Ti content in the range of 0.01% to 0.45%. A Ti content of 0.10% or higher is more preferred. Furthermore, a Ti content of 0.40% or less is even more preferred.

[0076] Nb: 0.01~0.60%

[0077] Nb, like Ti, is an element that suppresses sharpening by combining with C and N. These effects are achieved when the Nb content is 0.01% or higher. On the other hand, if the Nb content is greater than 0.60%, processability decreases. Therefore, in the case of Nb content, the Nb content is preferably in the range of 0.01% to 0.60%. The Nb content is more preferably 0.10% or higher. Furthermore, the Nb content is more preferably 0.40% or lower.

[0078] V: 0.01~0.30%

[0079] V, like Nb and Ti, combines with C and N contained in steel to suppress the decrease in corrosion resistance (sharpening). This effect is achieved when the V content is 0.01% or more. On the other hand, if the V content is greater than 0.30%, the workability decreases. Therefore, when V is present, the V content is preferably in the range of 0.01% to 0.30%. The V content is more preferably 0.20% or less, more preferably 0.15% or less, and even more preferably 0.10% or less.

[0080] It should be noted that the components other than those mentioned above are Fe and unavoidable impurities.

[0081] [Other metallic forms (Cr+Fe)] / [Metallic forms (Cr+Fe)]: Below 8.0

[0082] As described above, on the surface of chromium-containing steel sheets, by controlling the ratio of [non-metallic (Cr+Fe)] to [metallic (Cr+Fe)] to 8.0 or less, contaminants are sufficiently removed, resulting in a further reduction in interfacial resistance. Therefore, the ratio of [non-metallic (Cr+Fe)] to [metallic (Cr+Fe)] is preferably 8.0 or less. More preferably, it is 7.0 or less, and even more preferably 6.0 or less.

[0083] Here, [non-metallic forms (Cr+Fe)] / [metallic forms (Cr+Fe)] refers to the ratio of the total amount of Cr and Fe existing in non-metallic forms to the total amount of Cr and Fe existing in metallic forms on the surface of a chromium-containing steel plate. Furthermore, [metallic forms (Cr+Fe)] and [non-metallic forms (Cr+Fe)] are the total atomic concentrations of Cr and Fe existing in metallic forms and the total atomic concentrations of Cr and Fe existing in non-metallic forms, respectively, as determined by X-ray photoelectron spectroscopy analysis of the surface of the chromium-containing steel plate. Non-metallic forms manifest as oxides and hydroxides. Specifically, for Cr, examples include CrO2, Cr2O3, CrOOH, Cr(OH)3, and CrO3. For Fe, examples include FeO, Fe3O4, Fe2O3, and FeOOH. It should be noted that there is no particular limitation on the lower limit of [non-metallic forms (Cr+Fe)] / [metallic forms (Cr+Fe)], but if it is excessively lowered, the thickness of the chromium-containing steel plate becomes uneven. That is, the material is thinned by localized etching, making it prone to breakage. The ratio of [non-metallic form (Cr+Fe)] to [metallic form (Cr+Fe)] is preferably 2.0 or higher.

[0084] Here, the ratio of [non-metallic form (Cr+Fe)] to [metallic form (Cr+Fe)] is calculated as follows: X-ray photoelectron spectroscopy (XPS) is used to measure the surface of the steel plate. For the obtained Cr peaks, the peaks of Cr existing in metallic form are separated from the peaks of Cr existing in non-metallic forms. Similarly, for the obtained Fe peaks, the peaks of Fe existing in metallic form are also separated from the peaks of Fe existing in non-metallic forms. The atomic concentrations of Cr and Fe existing in non-metallic forms, calculated in this way, are then divided by the total atomic concentrations of Cr and Fe existing in metallic form.

[0085] Specifically, a 10mm square sample was cut from a steel plate. Using an Al-Kα monochromatic X-ray source and an X-ray photoelectron spectrometer (ULVAC PHI X-tool) at a 45-degree angle, the peaks of Cr and Fe were separated into those existing in metallic form and those existing in non-metallic form. The atomic concentrations of Cr and Fe existing in non-metallic forms were calculated by dividing the sum of their atomic concentrations by the sum of their atomic concentrations in metallic form. It should be noted that peak separation was performed by removing the spectral background using the Shirley method and utilizing the Gauss-Lorentz composite function (Lorentz function ratio: 30%).

[0086] The thickness of the chromium-containing steel sheet is preferably 50 μm or less. If the sheet thickness is 50 μm or less, it is easier to suppress the increase in battery weight. A thickness of 30 μm or less is more preferable. Furthermore, the thickness of the chromium-containing steel sheet is preferably 5 μm or more. If the sheet thickness is 5 μm or more, it helps to suppress a significant decrease in the production efficiency of the chromium-containing steel sheet and also controls the increase in manufacturing costs.

[0087] (2) Manufacturing method of chromium-containing steel plate for current collector of non-aqueous electrolyte secondary battery

[0088] Next, a method for manufacturing a chromium-containing steel sheet for a current collector of a non-aqueous electrolyte secondary battery according to one embodiment of the present invention will be described. The method for manufacturing a chromium-containing steel sheet for a current collector of a non-aqueous electrolyte secondary battery according to one embodiment of the present invention includes a preparation step of a chromium-containing steel sheet as a blank (a blank chromium-containing steel sheet), a first impregnation treatment step, and a second impregnation treatment step.

[0089] [Preparation process for chromium-containing steel sheet billets]

[0090] The preparation process for chromium-containing steel billets is the process of preparing chromium-containing steel billets as billets. There are no particular limitations on the chromium-containing steel billet material; for example, any chromium-containing steel billet material with the composition described above can be prepared as follows.

[0091] That is, a steel billet with the composition described above is hot-rolled into a hot-rolled sheet, which is then subjected to hot-rolled annealing and pickling as needed, and finally cold-rolled to obtain a cold-rolled sheet with the desired thickness. For example, when producing a chromium-containing steel sheet with a final thickness of 10 μm, cold-rolled annealing is performed as needed during the above-mentioned cold rolling process, and further cold rolling is performed to the final thickness.

[0092] It should be noted that there are no particular limitations on the conditions for hot rolling, cold rolling, hot-rolled sheet annealing, and cold-rolled sheet annealing; conventional methods can be used. Additionally, pickling can be performed after cold-rolled sheet annealing. Furthermore, cold-rolled sheet annealing can also be bright annealing. Prepare the steel sheet manufactured as described above as the billet for chromium-containing steel sheet.

[0093] [First Immersion Treatment (Etching Process)]

[0094] The chromium-containing steel sheet blank prepared as described above is subjected to a first immersion treatment (etching treatment). The first immersion treatment is performed by using an acidic aqueous solution containing hydrogen peroxide: 0.1 to 5.0% by mass, copper ions: 1.0 to 10.0% by mass, and halide ions: 5.0 to 20.0% by mass, with a pH of less than 1.0, and the treatment temperature and treatment time are 30 to 50°C and 40 to 90 seconds, respectively.

[0095] By implementing the above etching process, the amount of chromium-containing steel sheet dissolved in the billet can be precisely controlled, thereby controlling the shape of the uneven structure formed on the surface of the steel sheet.

[0096] The amount of chromium-containing steel sheet dissolved during etching can be controlled by adjusting the type of etching solution, temperature, and processing time. During etching, the steel sheet surface has easily soluble and difficult-to-dissolve areas, resulting in uneven dissolution. If the dissolution is insufficient, overall dissolution will not occur, thus failing to significantly increase Sa. If the dissolution is within the optimal range, the easily soluble areas will be etched, resulting in a significant increase in Sa, sharpening of the peak tips, and a positive Ssk value (greater than 0). Conversely, excessive dissolution, while increasing Sa, will also etch the difficult-to-dissolve areas, causing the peak tips to flatten and resulting in a negative Ssk value (less than 0).

[0097] Hydrogen peroxide: 0.1–5.0% by mass

[0098] If the hydrogen peroxide concentration in the treatment solution is less than 0.1% by mass, continuous etching cannot be performed due to reduced removal capacity of copper-containing products precipitated on the steel plate surface. On the other hand, if the hydrogen peroxide concentration is greater than 5.0% by mass, its effect saturates. Therefore, the hydrogen peroxide concentration is preferably 0.1% to 5.0% by mass.

[0099] Copper ions: 1.0–10.0% by mass

[0100] If the concentration of copper ions in the processing solution is less than 1.0% by mass, the desired uneven shape cannot be obtained due to decreased etching ability. On the other hand, if the concentration of copper ions is greater than 10.0% by mass, the amount of product adhering to the steel plate surface increases, and even with a second immersion treatment in the subsequent process, stains and the like cannot be completely removed. Therefore, the concentration of copper ions is preferably 1.0 to 10.0% by mass. More preferably, the concentration of copper ions is 2.0% by mass or more, and even more preferably 5.0% by mass or more.

[0101] Halide ions: 5.0–20.0% by mass

[0102] If the concentration of halide ions in the processing solution is less than 5.0% by mass, the passivation film on the steel plate surface cannot be sufficiently destroyed, and etching cannot be performed adequately. On the other hand, if the concentration of halide ions is greater than 20.0% by mass, local pitting corrosion is accelerated, potentially leading to perforation in the steel plate. Therefore, the concentration of halide ions is preferably 5.0 to 20.0% by mass. The concentration of halide ions is further preferably 10.0% by mass or more. In addition, the concentration of halide ions is further preferably 15% by mass or less. There is no particular limitation on the type of halide ion source; for example, hydrogen halides or alkali metal halides are preferred, and hydrochloric acid or sodium chloride are more preferred. Chloride ions are preferred as halide ions.

[0103] pH: below 1.0

[0104] If the pH of the treatment solution is greater than 1.0, the desired uneven texture cannot be formed on the steel plate surface due to decreased etching ability. Therefore, the pH of the treatment solution is set to 1.0 or lower. It should be noted that the pH of the treatment solution is preferably lower, more preferably 0.1 or lower.

[0105] The above-mentioned treatment solution (aqueous solution) can be prepared by stirring an aqueous solution of hydrogen peroxide, a copper compound that provides copper ions, a halide that provides halide ions, and water until homogeneous.

[0106] Processing temperature (temperature of the processing solution): 30~50℃

[0107] If the processing temperature is below 30°C, the etching ability decreases and the processing time increases. On the other hand, if the processing temperature is above 50°C, the stability of the processing solution decreases. Therefore, the processing temperature is set between 30°C and 50°C.

[0108] Processing time (immersion time): 40–90 seconds

[0109] If the processing time is less than 40 seconds, sufficient etching amount cannot be obtained. On the other hand, if the processing time is greater than 90 seconds, the required uneven structure cannot be formed on the steel plate surface. Moreover, productivity decreases. Therefore, the processing time is set to 40 to 90 seconds. It should be noted that since the etching amount varies depending on the type of chromium-containing steel, it is more preferable to adjust the processing time within the range of 40 to 90 seconds, depending on the type of steel.

[0110] There are no special restrictions on conditions other than those mentioned above; conventional methods are acceptable. It should be noted that while the above description exemplifies immersing the chromium-containing steel billet in an aqueous solution used as the treatment liquid, any contact between the billet and the aqueous solution, such as dripping or spraying, is also acceptable. In these cases, the treatment time is the contact time between the chromium-containing steel billet and the aqueous solution.

[0111] [Second Immersion Treatment (Stain Removal Treatment) Step]

[0112] Following the first impregnation treatment described above, the chromium-containing steel sheet billet undergoes a further second impregnation treatment.

[0113] (A) Immersion treatment was performed using an acidic aqueous solution containing hydrogen peroxide at temperatures ranging from 30 to 60°C and for durations ranging from 5 to 120 seconds.

[0114] (B) Immersion treatment using an aqueous solution containing nitric acid, with a treatment temperature of 30–60°C and a treatment time of 5–120 seconds, or

[0115] (C) Implement the combined processing of (A) and (B) above.

[0116] This can dissolve (remove) the deposits such as stains formed during the first immersion treatment (etching treatment).

[0117] That is, after the first impregnation treatment, contaminants (a highly resistive mixture mainly composed of C, N, S, O, Fe, Cr, Ni, and Cu) are generated on the surface of the steel plate. If these (contaminants, etc.) remain, even if the desired uneven structure is obtained on the surface of the steel plate, it may still become a cause of increased interfacial resistance. Based on this, after the first impregnation treatment described above, by performing the above-described treatments (A), (B), or (C), the aforementioned contaminants are removed, and a stable reduction in interfacial resistance is obtained. Here, an acidic aqueous solution containing hydrogen peroxide can be cited as an example, which is a mixed aqueous solution of hydrogen peroxide and sulfuric acid. In addition, an aqueous solution containing nitric acid can be cited as an example, which is an aqueous solution of nitric acid.

[0118] Furthermore, when using a mixed aqueous solution of hydrogen peroxide and sulfuric acid, the concentration of hydrogen peroxide is preferably 0.5 to 10.0% by mass, and the concentration of sulfuric acid is preferably 1.0 to 10.0% by mass. Additionally, when using an aqueous solution of nitric acid, the concentration of nitric acid is preferably 1.0 to 40.0% by mass. It should be noted that in the mixed aqueous solution of hydrogen peroxide and sulfuric acid, the components other than hydrogen peroxide and sulfuric acid, and in the aqueous solution of nitric acid, the components other than nitric acid, are essentially water.

[0119] Furthermore, in either case (A) or (B) above, the processing temperature (temperature of the processing solution) of the second impregnation treatment is preferably 30 to 60°C.

[0120] Furthermore, the longer the treatment time (immersion time), the more effective the removal of stains, etc., but if it is too long, the effect will saturate and productivity will decrease. Therefore, in either case (A) or (B) above, the treatment time is preferably 5 to 120 seconds. The treatment time is more preferably 30 seconds or more. In addition, the treatment time is more preferably 90 seconds or less. It should be noted that in the treatment (C), there is no particular limitation on the order of the treatments (A) and (B) above. Preferably, the treatment (C) is performed after the treatment (A), followed by the treatment (B).

[0121] In addition, as described in (B) or (C) above, by using an aqueous solution containing nitric acid for immersion treatment, it is possible to more effectively dissolve (remove) deposits such as stains formed during the first immersion treatment (etching treatment) and further improve the effect of reducing interface resistance.

[0122] It should be noted that during the second immersion treatment, if the surface of the chromium-containing steel sheet to be treated is wiped with a non-woven cloth or similar material as needed, it is easier to remove stains and thus reliably achieve a further reduction in interfacial resistance. Furthermore, although the above description mentions immersing the chromium-containing steel sheet in an aqueous solution used as the treatment liquid, any contact between the chromium-containing steel sheet and the aqueous solution, such as dripping or spraying, is also acceptable. In these cases, the treatment time is the contact time between the chromium-containing steel sheet and the aqueous solution. In addition to continuous treatment of the steel strip, the second immersion treatment can also be performed after the strip has been shaped into a current collector.

[0123] Example

[0124] A chromium-containing steel billet with a thickness of 10 μm and the composition (remaining as Fe and unavoidable impurities) as described in Table 1 was prepared. Next, the prepared chromium-containing steel billet was subjected to a first immersion treatment (etching treatment) and a second immersion treatment (stain removal treatment) according to the conditions shown in Tables 2 and 3 to obtain a chromium-containing steel billet for the current collector of a non-aqueous electrolyte secondary battery (samples No. 1, 3-8, 11-17). Additionally, a steel billet without the above treatment was directly used as sample No. 2 ("-" in the manufacturing conditions of Tables 2 and 3 indicates that this treatment was not performed). It should be noted that the compositions of the treatment solutions used in the first immersion treatment (etching solutions A1-A4) and the aqueous solutions used in the second immersion treatment (stain removal solutions B1-B2, E) are as follows.

[0125] <Etching Solution A1>

[0126] Hydrogen peroxide: 0.2% by mass

[0127] Copper ions: 1.5% by mass

[0128] Chloride ions: 10.0% by mass

[0129] Remaining portion: water

[0130] pH: 0.05

[0131] It should be noted that the copper ions and chlorides originate from copper sulfate pentahydrate and hydrochloric acid, respectively.

[0132] <Etching Solution A2>

[0133] Hydrogen peroxide: 0.3% by mass

[0134] Copper ions: 9.0% by mass

[0135] Chloride ions: 10.0% by mass

[0136] Remaining portion: water

[0137] pH: 0.05

[0138] It should be noted that the copper ions and chlorides originate from copper sulfate pentahydrate and hydrochloric acid, respectively.

[0139] <Etching Process A3>

[0140] Hydrogen peroxide: 2.0% by mass

[0141] Copper ions: 2.0% by mass

[0142] Chloride ions: 10.0% by mass

[0143] Remaining portion: water

[0144] pH: 0.05

[0145] It should be noted that the copper ions and chlorides originate from copper sulfate pentahydrate and hydrochloric acid, respectively.

[0146] <Etching Solution A4>

[0147] Hydrogen peroxide: 0.3% by mass

[0148] Copper ions: 2.0% by mass

[0149] Chloride ions: 15.0% by mass

[0150] Remaining portion: water

[0151] pH: 0.05

[0152] It should be noted that the copper ions and chlorides originate from copper sulfate pentahydrate and hydrochloric acid, respectively.

[0153] <Stain Removal Solution B1>

[0154] Hydrogen peroxide: 2.0% by mass

[0155] Sulfuric acid: 4.0% by mass

[0156] Remaining portion: water

[0157] <Stain Removal Solution B2>

[0158] Hydrogen peroxide: 3.0% by mass

[0159] Sulfuric acid: 6.0% by mass

[0160] Remaining portion: water

[0161] <Stain Removal Solution E>

[0162] Nitric acid: 30% by mass

[0163] Remaining portion: water

[0164] For the chromium-containing steel sheet manufactured as described above, Sa and Ssk were determined according to ISO 25178. It should be noted that a laser microscope (Keyence VK-X250 / X260) was used for the measurement. Specifically, a sample was taken from the manufactured chromium-containing steel sheet, and the surface shape data of a 50μm × 50μm area on each side of the sample was measured using the aforementioned laser microscope with a 150x objective lens. The obtained data was analyzed using the instrument's accompanying analysis software, "Multi-file Analysis Application," to determine Sa and Ssk for each side of the sample. It should be noted that before analyzing Sa and Ssk, image processing was performed, including interpolation to remove light intensity outside the threshold range, smoothing using a Gaussian function, and setting segmentation levels to remove noise. Then, a reference plane was set as the measurement reference, and the surface was corrected to a plane through two surface corrections. Furthermore, the filter type was a Gaussian filter with a specified cutoff wavelength of 0.5μm. The measurement results are shown in Tables 2 and 3. It should be noted that since Sa and Ssk are essentially the same on both sides of any specimen, the values ​​of Sa and Ssk measured on one side of the specimen are used as the representative values ​​and recorded in Tables 2 and 3.

[0165] In addition, for samples No. 11 to 17, the ratio of [non-metallic form (Cr+Fe)] to [metallic form (Cr+Fe)] was calculated using the method described above. The results are also recorded in Table 3.

[0166] In addition, using chromium-containing steel sheets manufactured as described above, corrosion resistance, electrode resistance, rate performance, and cycle performance are evaluated according to the following criteria.

[0167] [Corrosion Resistance Evaluation]

[0168] The corrosion resistance under battery conditions was evaluated by observing the current density of the chromium-containing steel plate during the potential scan process in the working electrode using the chromium-containing steel plate manufactured above, and in the electrolyte (1M LiPF6, ethylene carbonate: diethyl carbonate = 1:1 (volume ratio)) within the following potential range.

[0169] It should be noted that the above evaluation was performed by raising the potential from the initial immersion potential (the immersion potential at the start of the experiment) to 5.0V, then lowering the potential to 0.0V, and then raising the potential back to the initial immersion potential until the potential reached the initial immersion potential.

[0170] Based on this evaluation, it can be determined that if the current density is low, corrosion resistance in a battery environment can be ensured because no corrosion products or pitting are generated on the surface of the chromium-containing steel plate. It should be noted that the measurement was conducted in an argon atmosphere with a dew point below -70°C at 25°C.

[0171] Potential scanning method

[0172] Potential scan range: Initial immersion potential (immersion potential at the start of the experiment) → 5.0V → 0.0V → Initial immersion potential

[0173] The potential is V(vs.Li / Li) relative to the counter electrode Li metal foil. + )

[0174] Scan speed: 5mV / s

[0175] It should be noted that the evaluation criteria for corrosion resistance are as follows. The evaluation results are shown in Tables 2 and 3.

[0176] ○ (Pass): The absolute value of the maximum current density is 100 μA / cm 2 the following

[0177] × (Unacceptable): The absolute value of the maximum current density is greater than 100 μA / cm². 2

[0178] A coin-shaped battery with a non-aqueous electrolyte lithium-ion secondary battery having the following battery configuration was fabricated, and its electrode resistance, rate performance, and cycle performance were evaluated under the following conditions.

[0179] [Electrode fabrication and battery cell fabrication]

[0180] (Battery Composition)

[0181] Coin-shaped cell (positive electrode area: 15mmΦ, negative electrode area: 16mmΦ)

[0182] Positive electrode active material: LiNi 0.6 Mn0.2 Co 0.2 O2 (Ni:Mn:Co = 6:2:2 (atomic ratio))

[0183] Positive conductive agent: Acetylene black

[0184] Positive electrode binder: polyvinylidene fluoride

[0185] It should be noted that the positive electrode ratio is active material: conductive agent: binder = 94:3:3.

[0186] Positive current collector: Al foil

[0187] Negative electrode active material: natural graphite

[0188] Negative electrode thickener: Carboxymethyl cellulose

[0189] Negative electrode binder: Styrene-butadiene rubber

[0190] It should be noted that the negative electrode ratio is active material: thickener: binder = 98:1:1.

[0191] Negative current collector: Each of the chromium-containing steel plates manufactured above

[0192] Electrolyte: 1M LiPF6, ethylene carbonate: methyl ethyl carbonate: dimethyl carbonate = 1:1:1 (volume ratio), vinylene carbonate (1 wt%)

[0193] Spacer: Polypropylene spacer

[0194] [Electrode Resistance Evaluation]

[0195] After the above electrode fabrication, the thickness-direction resistance of the negative electrode (a mixture of negative electrode active material, negative electrode thickener, and negative electrode binder coated on the surface of the negative electrode current collector) was measured using the 4-probe method (Mitsubishi Chemical, Loresta EP low resistivity meter, using BSP probes (4 probes)). The evaluation area was set to 5 cm². 2 The measurement was performed three times and the average value was taken.

[0196] It should be noted that the evaluation criteria for electrode resistance are as follows. The evaluation results are shown in Tables 2 and 3.

[0197] ◎(Pass): Electrode resistance value below 0.35Ω

[0198] ○ (Pass): Electrode resistance value greater than 0.35Ω and less than 0.60Ω

[0199] × (Unacceptable): Electrode resistance value greater than 0.60Ω

[0200] [Rate Magnification Characteristics Evaluation]

[0201] After manufacturing a coin-shaped battery with the above-described non-aqueous electrolyte lithium-ion secondary battery structure, rate characteristics were evaluated under the following conditions.

[0202] (Experimental conditions)

[0203] Charging: After reaching 4.2V with constant current and constant voltage at 0.1C, charging ends when the current reaches 0.01C.

[0204] Pause: 10 minutes

[0205] Discharge: Ends when the 1C constant current reaches 2.5V, or ends when the 5C constant current reaches 2.5V. Temperature: 25℃

[0206] It should be noted that the evaluation criteria for rate capability are as follows. The evaluation results are shown in Tables 2 and 3.

[0207] ◎(Qualified): The ratio of discharge capacity at 5C to discharge capacity at 1C (capacity retention) is 60% or higher.

[0208] ○ (Qualified): The ratio of discharge capacity at 5C to discharge capacity at 1C is 50% or more and less than 60%.

[0209] × (Unacceptable): The ratio of discharge capacity at 5C to discharge capacity at 1C is less than 50%.

[0210] The ratio (%) of discharge capacity at 5C to discharge capacity at 1C is calculated as 100 × (discharge capacity at 5C / discharge capacity at 1C).

[0211] [Cyclic Performance Evaluation]

[0212] After manufacturing a coin-shaped battery with the above-described non-aqueous electrolyte lithium-ion secondary battery structure, the cycle characteristics were evaluated under the following conditions.

[0213] (Experimental conditions)

[0214] Charging: After reaching 4.2V with constant current and constant voltage at 1C, charging ends when the current reaches 0.1C.

[0215] Pause: 10 minutes

[0216] Discharge: Ends when the 1C constant current reaches 2.5V.

[0217] Temperature: 25℃

[0218] One cycle: Charge → Pause → Discharge → Pause

[0219] Number of loops: 300

[0220] It should be noted that the evaluation criteria for cycle characteristics are as follows, and the evaluation results are shown in Tables 2 and 3.

[0221] ◎(Qualified): Capacity retention rate of over 96% after 300 discharge cycles

[0222] ○ (Pass): Capacity retention rate after 300 discharge cycles is above 90% and less than 96%.

[0223] × (Unacceptable): Capacity retention rate less than 90% after 300 discharge cycles.

[0224] The 300-cycle discharge capacity retention rate (%) is calculated as 100 × (300-cycle discharge capacity / 1-cycle discharge capacity).

[0225]

[0226]

[0227]

[0228] Tables 2 and 3 illustrate the following.

[0229] (a) In any of the invention examples, the desired rate capability and cycle capability can be obtained by reducing the electrode resistance.

[0230] (b) On the other hand, in comparative examples No. 2, 3 and 5, the electrode resistance was not sufficiently reduced, and the desired rate performance and cycle performance were not obtained.

[0231] In addition, the following information is provided in Table 3.

[0232] (c) In the invention example where the treatment in an aqueous solution containing nitric acid was added, the electrode resistance was significantly reduced, and the rate performance and cycle performance were further improved.

Claims

1. A chromium-containing steel plate for the current collector of a non-aqueous electrolyte secondary battery, comprising, by mass%, C: 0.001–0.100%, Si: 0.01–2.00%, Mn: 0.01–2.00%, P: less than 0.050%, S: less than 0.010%, Cr: 10.00–32.00%, Ni: 0.01–30.00%, Al: 0.001–0.500%, and N: less than 0.100%, with the remainder being Fe and unavoidable impurities. Furthermore, the parameter Sa specified in ISO 25178 is 0.15 μm to 0.50 μm, and the parameter Ssk specified in ISO 25178 is greater than 0. [Other forms (Cr+Fe)] / [Metallic forms (Cr+Fe)] is 8.0 or less. in, [Other forms of Cr+Fe] / [Metallic forms of Cr+Fe] refers to the ratio of the total amount of Cr and Fe existing in non-metallic forms to the total amount of Cr and Fe existing in metallic forms on the surface of the chromium-containing steel plate.

2. The chromium-containing steel plate for the current collector of the non-aqueous electrolyte secondary battery according to claim 1, wherein the composition further comprises one or more of the following by mass percent: Mo: 0.01-2.50%, Cu: 0.01-0.80%, Ti: 0.01-0.45%, Nb: 0.01-0.60%, V: 0.01-0.30%.

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