Cladding, current collector for bipolar batteries, bipolar batteries

Abstract

To provide a cladding having corrosion resistance, conductivity, and thermal conductivity coefficient, while holding flatness caused by shape correction and strength for shape maintaining, and a power collector of a bipolar battery using the same, and a bipolar battery.SOLUTION: There is provided a cladding including: an Al layer; and a stainless steel layer joined to the Al layer, where the Al layer is a pure Al layer whose Al amount in mass% is 99% or greater, the stainless steel layer is a ferritic stainless steel layer including 13.0-19.0% of Cr and 0-0.5% of Mo in mass%, tension strength of the cladding is 1.20 times or greater and 2.40 times or smaller of an σ value indicated by a following formula 1. The formula is:σ=α×x+β(1-x), in the formula 1, α, β, x indicate respectively, tension strength of the Al layer in a finished and annealed state, tension strength of the stainless steel layer in the finished and annealed state, and a ratio of a thickness of the Al layer with respect to a thickness of the cladding.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] This disclosure relates to cladding, current collectors for bipolar batteries, and bipolar batteries. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries are useful because they can operate at higher voltages compared to secondary batteries using aqueous electrolytes. However, as the operating voltage increases, the operating environment for the current collector becomes more harsh, and the risk of metal corrosion and melting is expected to increase. Therefore, the materials used in the current collector require excellent corrosion resistance.

[0003] In recent years, development of bipolar batteries has progressed with the aim of further increasing the capacity of secondary batteries. A bipolar battery is a battery that uses a current collector having a metal plate, a positive electrode formed on one side of the metal plate, and a negative electrode formed on the other side of the metal plate. For example, Patent Document 1 discloses a bipolar battery using stainless steel in which the Cr content is specified as 16-26% and the Mo content as 0.5-7% by mass in order to ensure the corrosion resistance required for the current collector. Patent Document 1 states that by specifying the Cr and Mo content of the stainless steel used as the current collector, corrosion on the positive electrode side of the current collector can be suppressed, and the durability and lifespan characteristics can be improved.

[0004] On the other hand, current collectors using cladding containing Al have also been proposed from the viewpoint of electrical conductivity and thermal conductivity. Cladding is a metallic material made by layering metals with different properties, and it is possible to combine the characteristics of each constituent metal. Cladding current collectors containing Al as a constituent material can combine the excellent electrical conductivity and thermal conductivity for dissipating Joule heat of Al with the properties of the other layered metals.

[0005] For example, Patent Document 2 discloses a bipolar battery that uses aluminum foil or an aluminum-containing cladding as a current collector. Furthermore, Patent Document 3 discloses a cladding in which Al and Cu are laminated with a core material layer having a Young's modulus of 150 GPa or more in between. Furthermore, Patent Document 4 proposes a method for welding current collectors with low electrical resistance, such as Al or Cu, to current collector terminals, in which only the portion where the current collector and current collector terminal are joined by welding is clad with stainless steel, which has high resistive heating. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2007-242424 [Patent Document 2] Patent No. 5205713 [Patent Document 3] Japanese Patent Publication No. 2017-191794 [Patent Document 4] Japanese Patent Publication No. 2002-260670 [Non-patent literature]

[0007] [Non-Patent Document 1] Seung-Taek Myung, Yusuke Sasaki, Shuhei Sakurada, Yang-Kook Sun and Hitoshi Yashiro: Electrochimica Acta, 55(2009), 288-297 [Overview of the project] [Problems that the invention aims to solve]

[0008] As described above, several inventions have been made to date regarding current collectors with corrosion resistance capable of withstanding harsh operating environments. However, the current collectors used in high-performance bipolar batteries, which have been developed in recent years, have several challenges that have not been solved by the above inventions.

[0009] The stainless steel bipolar current collector described in Patent Document 1 has poor electrical and thermal conductivity, and the Joule heat generated by its high electrical resistance is difficult to dissipate. The aluminum-clad current collector described in Patent Document 2 possesses the low electrical resistance and high thermal conductivity inherent in aluminum. However, it has low strength, and through-holes may form in the current collector during the pressing process to ensure close contact between the active material and the current collector. Furthermore, in the case of large-area electrode plates, problems such as deformation of the electrode plates can occur, leading to short circuits in the battery or variations in performance within the electrode plate surface. The current collector described in Patent Document 4 is also a current collector in which only the joint portion of the current collector terminal is clad, but the main part of the electrode plate that joins with the active material remains the original material, and thus has the same problems as in Patent Document 2.

[0010] On the other hand, while Patent Document 3 increases strength by providing a core material layer, it is preferable to use a Ni-Nb alloy, which requires homogeneous dissolution of high-melting-point Nb, thus increasing material costs. Furthermore, the Cu bonded to the Al layer via the core material layer is highly reactive with ions, raising concerns about corrosion when using sulfide-based solid electrolytes, which have been investigated in recent years.

[0011] Thus, the current collector of a bipolar battery is required to possess corrosion resistance, conductivity, and thermal conductivity, while also having the strength to maintain its shape and ensuring flatness through shape correction. This improves the battery's capacity retention and further suppresses the rise in battery temperature.

[0012] On the other hand, fields that require corrosion resistance, conductivity, and thermal conductivity, while also needing strength to maintain shape and flatness through shape correction, are not limited to current collectors for bipolar batteries. They are also required for heat dissipation components used in thermal management of machinery, housings and internal electronic components of electronic equipment, and structural components.

[0013] Therefore, the object of the present invention is to provide a cladding that has corrosion resistance, conductivity and thermal conductivity, while ensuring strength for maintaining its shape and flatness through shape correction, as well as a current collector for a bipolar battery and a bipolar battery that utilize this cladding.

Means for Solving the Problem

[0014] The means for solving the problem include the following aspects. <1> Having an Al layer and a stainless steel layer joined to the Al layer, The Al layer is a pure Al layer with an Al content of 99% or more by mass%, The stainless steel layer is a ferritic stainless steel layer containing, by mass%, Cr: 13.0 to 19.0% and Mo: 0 to 0.5%, A clad in which the tensile strength of the clad is 1.20 times or more and 2.40 times or less of the σ value represented by the following formula 1. σ = α × x + β(1 - x) ··· Formula 1 In Formula 1, here, α, β, and x respectively represent the tensile strength of the Al layer in the annealed state, the tensile strength of the stainless steel layer in the annealed state, and the ratio of the thickness of the Al layer to the thickness of the clad. <2> The clad according to <1>, wherein the Vickers hardness HV on the surface of the clad on the Al layer side is 60 or less. <3> The clad according to <1> or <2>, wherein the thickness ratio of the Al layer to the stainless steel layer (the Al layer / the stainless steel layer) is 4 / 6 to 8 / 2. <4> The clad according to any one of <1> to <3>, wherein the thickness of the clad is 600 μm or less. <5> The clad according to any one of <1> to <4> for a current collector of a bipolar battery. <6> A current collector of a bipolar battery comprising the clad according to any one of <1> to <5>. <7> The current collector of the bipolar battery according to <6>, having a return of the Al layer in the direction of the Al layer from the bonding interface between the Al layer and the stainless steel layer at the end of the current collector. <8> A bipolar battery having the current collector of the bipolar battery according to <6> or <7>.

Advantages of the Invention

[0015] According to the present invention, it is possible to provide a cladding that has corrosion resistance, conductivity and thermal conductivity, while ensuring strength for maintaining its shape and flatness through shape correction, as well as a current collector for a bipolar battery and a bipolar battery utilizing the cladding. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic diagram showing the end of the cladding of the present disclosure, which has a return of the Al layer in the direction of the stainless steel layer from the bonding interface between the Al layer and the stainless steel layer. [Figure 2] This is a schematic diagram showing the end portion of the cladding of the present disclosure, which has a return of the Al layer in the direction of the Al layer from the bonding interface between the Al layer and the stainless steel layer. [Figure 3] This is a schematic diagram showing an example of a bipolar battery according to the present disclosure. [Figure 4] This is a schematic diagram showing a simulated cell device for laboratory evaluation of the bipolar battery fabricated in the example. [Modes for carrying out the invention]

[0017] An example of this disclosure is described below. In this disclosure, the percentage (%) for the content of each element in the chemical composition refers to "mass percent". When the lower limit for the content of each element in the chemical composition is stated as "0," it means that the element is an optional component and does not need to be included. A numerical range represented using "~" means a range that includes the numbers written before and after "~" as the lower and upper limits, respectively. In numerical ranges expressed using "~", if the numbers before and after "~" are not preceded by "greater than" or "less than", it means the range includes those numbers as the lower and upper limits. Conversely, if the numbers before and after "~" are preceded by "greater than" or "less than", it means the range does not include those numbers as the lower or upper limit. In the numerical ranges described in stages, the upper limit of one stage may be replaced with the upper limit of another stage, or with the value shown in the example. Similarly, the lower limit of one stage may be replaced with the lower limit of another stage, or with the value shown in the example. The term "process" includes not only independent processes, but also any process that cannot be clearly distinguished from other processes, as long as its intended purpose is achieved.

[0018] "Plate thickness" is the average value of three measurements taken by observing the cross-section along both the rolling direction and the thickness direction.

[0019] <Clad> The cladding of this disclosure comprises an Al layer and a stainless steel layer bonded to the Al layer. The Al layer is a pure Al layer with an Al content of 99% or more by mass. The stainless steel layer is a ferritic stainless steel layer containing Cr: 13.0-19.0% and Mo: 0-0.5% by mass. The tensile strength of the cladding is between 1.20 and 2.40 times the σ value shown in Equation 1 below. σ = α × x + β(1-x) ... Equation 1 In Equation 1, α, β, and x represent the tensile strength of the annealed Al layer, the tensile strength of the annealed stainless steel layer, and the ratio of the thickness of the Al layer to the thickness of the cladding, respectively.

[0020] The cladding of this disclosure, through the above configuration, possesses corrosion resistance, electrical conductivity, and thermal conductivity, while ensuring strength for shape retention and flatness through shape correction. The cladding of this disclosure was discovered through the following findings.

[0021] First, the inventors diligently investigated and aimed to solve the following problems that remained unresolved with conventional materials for current collectors in bipolar batteries.

[0022] (i) If ions are leached due to metal corrosion, they may precipitate on the negative electrode side and cause a short circuit between the positive and negative electrodes. Therefore, it is necessary to suppress metal dissolution due to corrosion in harsh corrosive environments that may occur during rapid and high-current charging. (ii) The heat generated by the resistance of the components that make up the battery can lead to excessive temperature rises in large bipolar batteries with poor heat dissipation, potentially creating a dangerous situation during use. (iii) There is a need for increased strength in current collectors. Conventional materials such as pure Al (Al with a tensile strength of about 80 MPa), pure Cu (Cu with a tensile strength of about 220 MPa), or cladding made by laminating these materials are insufficient in strength, and cladding with materials that have greater strength is necessary. (iv) However, if the strength is made excessively high through rolling or other processes, the sheet is prone to warping, making it difficult to correct the shape during finish rolling. Furthermore, even if sufficient flatness is ensured through shape correction, the pressure applied to the current collector may become uneven due to the expansion and contraction behavior of the battery during operation.

[0023] Next, the inventors, after diligent study, obtained the following findings.

[0024] (A) As shown in Patent Document 1, stainless steel is one of the currently proposed materials for current collectors. Stainless steel has a Cr2O3-based passive film on its surface, which ensures high corrosion resistance, but its structure and composition vary depending on the chemical composition of the stainless steel. It has been found that the corrosion resistance to non-aqueous solvents and non-aqueous electrolytes changes depending on the change in the passive film due to the chemical composition. Stainless steel is classified into several types based on its internal structure, including austenitic, ferritic, martensitic, austenitic-ferrite duplex, and precipitation-strengthened types. Experimental results show that, although the detailed mechanism is unknown, ferritic stainless steel with low Ni content tends to exhibit high corrosion resistance in lithium-ion secondary battery environments. On the other hand, according to Non-Patent Literature 1, generally, Cr2O3, the main component of the passivation film on stainless steel surfaces, is 4.7V vs Li / Li+ At this voltage, the reaction shown in equation 2 below occurs, destroying the passivation film on stainless steel and causing hexavalent Cr ions to leach out. Cr2O3 + 5Li2O → 2CrO4 2- +10Li + +6e - at 4.7V vs Li / Li + ...Formula 2 In other words, under the above voltage conditions, the surface of the stainless steel corrodes and metal ions leach out. If the amount of leached metal is large, the metal ions move through the electrolyte, deposit and grow as metal on the negative electrode side, potentially piercing the separator and causing a short circuit in the battery.

[0025] (B) When stainless steel is used alone, in addition to preventing the reaction in formula 2 above, measures are needed to address its relatively low electrical and thermal conductivity. For this purpose, cladding with Al, which has good electrical and thermal conductivity, is effective.

[0026] (C) Furthermore, in stainless steel, Al does not react under the high-potential conditions in which the reaction in Equation 2 occurs, and is therefore stable. For this reason, by cladding Al with stainless steel and orienting the Al layer toward the positive electrode side where the potential is high, the reaction in Equation 2 of the current collector can be suppressed.

[0027] (D) With the practical application of solid electrolytes and electrodes containing high-capacity Si compounds progressing, there is a need for current collectors with higher strength than Cu and Al. In this respect, stainless steel is stronger than Cu and Al and is effective in improving the strength of current collectors. On the other hand, if the strength is excessively high, it becomes difficult to correct the shape when applying cladding to the current collector. To achieve both the necessary strength to maintain the shape of the current collector and the necessary flatness through shape correction, it has been found that controlling the tensile strength of the cladding that forms the current collector is effective by using a combined law to determine the strength based on the tensile strength of the Al layer and stainless steel layer that make up the cladding after annealing. In other words, by setting the tensile strength of the cladding that serves as the current collector to a range of 1.20 to 2.40 times the strength calculated using the combined law based on the tensile strength of the Al layer and stainless steel layer in their annealed state (i.e., the σ value shown in Equation 1), it is possible to achieve both the necessary strength to maintain the shape of the current collector and the necessary flatness through shape correction of the current collector.

[0028] (E) In bipolar batteries, the current collector needs to maintain the battery structure with as uniform a pressure as possible, even if expansion or contraction occurs due to chemical reactions, heat generation, etc., during battery operation. When bipolar batteries are used for a long period of time, the pressure on the current collector may become uneven due to minute deformations caused by expansion and contraction, or due to external forces. In this case, if the current collector is rigid, the uneven pressure will remain, which may lead to a decrease in the battery's performance. In contrast, the Al cladding layer is flexible, and its surface shape changes to conform to the shape of the active material and solid electrolyte that deform due to expansion and contraction during battery operation, thereby equalizing the pressure applied to the battery.

[0029] Next, the inventors investigated whether the cladding, which has properties suitable for use as a current collector in bipolar batteries, could be applied to other fields. As a result, the inventors found that the cladding suitable for use as a current collector in bipolar batteries is also suitable for heat dissipation components used in thermal management of machinery, housings and internal electronic components of electronic equipment, structural components, and the like.

[0030] From the above findings, it was found that the cladding of this disclosure possesses corrosion resistance, electrical conductivity, and thermal conductivity while ensuring strength and flatness for maintaining its shape.

[0031] The details of the cladding described herein will be explained below, and as an example of the cladding described herein, the case in which the cladding described herein is applied to a current collector of a bipolar battery will be described below.

[0032] (1) Overall structure The cladding of this disclosure is a cladding having an Al layer and a stainless steel layer bonded to the Al layer. The stainless steel layer is bonded to one of the two opposing surfaces of the Al layer in the thickness direction. Furthermore, both the Al layer and the stainless steel layer have a passive coating formed on both opposing surfaces in the thickness direction.

[0033] (2) Stainless steel layer The material used for the stainless steel layer is limited to ferritic stainless steel, which has the most suppressed reaction in formula 2 and possesses corrosion resistance. Ferritic stainless steel is stainless steel with a body-centered cubic ferrite phase as its matrix. Representative examples include the steel grades specified in JIS G 4305:2015. Specific steel grades include SUS403, SUS410, SUS429, SUS430, SUS430J1L, and SUS430LX, as specified in the JIS standard.

[0034] (3) Chemical composition of the stainless steel layer: Cr: 13.0-19.0%, Mo: 0-0.5% Unlike corrosion in aqueous electrolytes, corrosion of stainless steel in non-aqueous electrolytes does not directly translate to improved corrosion resistance with increasing chromium (Cr) content. The actual reaction is the dissolution of the Cr passivation film, as shown in Equation 2 above. When the Cr content is high, a thick Cr passivation film is formed, leading to a large amount of Cr dissolution due to over-passivation corrosion. Therefore, in order to have a moderate level of corrosion resistance while minimizing the amount of chromium dissolved, the upper limit of the chromium content in the stainless steel layer is 19% or less. From the viewpoint of suppressing dissolution, the preferred upper limit of chromium content is 18% or less.

[0035] On the other hand, electrolytes may contain trace amounts of water, and if LiPF6 is present in a non-aqueous electrolyte, it may react with water to produce HF. Therefore, in order to ensure corrosion resistance against corrosion caused by trace amounts of HF generated from the electrolyte, the lower limit of the Cr content is 13% or more. From the viewpoint of corrosion resistance, the preferred lower limit of the Cr content is 13.5% or more.

[0036] Mo in stainless steel is generally an element that enhances corrosion resistance. This is because, during corrosion in aqueous solutions, when the passive film of Cr is destroyed by chloride ions, etc., Mo acts as an inhibitor and helps repair Cr2O3. On the other hand, when current collectors are used in non-aqueous electrolyte environments, unlike corrosion in aqueous solutions, the effect of Mo on corrosion resistance cannot be expected. Therefore, Mo does not need to be included in the stainless steel layer. In other words, the amount of Mo may be 0%. However, since Mo may be contained in raw materials such as scrap during stainless steel manufacturing, complete removal of Mo may be difficult. Therefore, the lower limit of the amount of Mo may be 0.01% or more, or 0.03% or more. Even when actively incorporating Mo, excessive amounts of Mo lead to increased electrical resistance and higher temperatures during battery operation. Furthermore, Mo is a very expensive alloying element. Therefore, the upper limit for Mo content is 0.5% or less. From the standpoint of alloy cost, the preferred upper limit for Mo content is 0.3% or less, and the more preferred upper limit for Mo content is 0.1% or less.

[0037] Here, Cu is 3.5V vs Li / Li + These are elements that dissolve at the above potential. Therefore, they are not elements that should be actively included in stainless steel layers. In other words, Cu does not need to be included in stainless steel layers. That is, the amount of Cu may be 0%. However, since copper (Cu) may be present in raw materials such as scrap from stainless steel manufacturing, complete removal of Cu can be difficult. Nevertheless, the upper limit for the amount of Cu is preferably 0.6% or less. From the viewpoint of corrosion resistance, a more preferable upper limit for the amount of Cu is 0.4% or less. Since complete removal of Cu may be difficult, the lower limit of the Cu content may be 0.01% or higher, or 0.03% or higher.

[0038] In order to maintain strength while improving corrosion resistance, electrical conductivity, and thermal conductivity, the chemical composition of the stainless steel layer is as follows: C: 0~0.15%, Si: 0~3.0%, Mn: 0~1.0%, P: 0-0.08% S: 0~0.01%, N: 0~0.02%, Cr: 13.0~19.0%, Mo: 0~0.5%, Cu: 0~0.6%, Ni: 0~1.0%, Ti: 0~0.8%, Nb: 0~1.0%, Al: 0-0.5% V: 0~0.5%, B: 0~0.0.01%, Sn: 0~0.05%, Ca: 0~0.005%, Mg: 0~0.005%, Co: 0~0.5%, REM: 0~0.1%, and Remainder: Fe and impurities It is preferable that the chemical composition consists of the following:

[0039] (4) Al layer Pure aluminum is a stable element in non-aqueous electrolyte environments when used as a positive electrode current collector. This is because the Al2O3 passivation film on its surface remains stable and does not dissolve even at high potentials, and even when it reacts with an electrolyte, an Al fluoride film is formed on the surface of the Al2O3 passivation film, and this Al fluoride film acts as a protective film together with the Al2O3 passivation film. For this reason, by combining pure aluminum with stainless steel to form a cladding and coating the positive electrode active material on the Al layer side, the drawback of stainless steel, which undergoes corrosion reactions at high potentials, can be overcome. On the other hand, the Al layer needs to be of high purity and not contain large amounts of elements such as Si, Mg, Mn, and Zn, which are at risk of dissolving when voltage is applied. Therefore, the Al layer is limited to a pure Al layer with an Al content of 99% or more.

[0040] (5) Tensile strength of the cladding The tensile strength of the cladding is between 1.20 and 2.40 times the σ value shown in Equation 1 below. σ = α × x + β(1-x) ... Equation 1 In Equation 1, α, β, and x represent the tensile strength of the annealed Al layer, the tensile strength of the annealed stainless steel layer, and the ratio of the thickness of the Al layer to the thickness of the cladding, respectively.

[0041] If the tensile strength of the cladding is less than 1.20 times the σ value, it may not be able to maintain its shape when used as a current collector. On the other hand, if the tensile strength of the cladding exceeds 2.40 times the σ value, it may become difficult to correct the shape to ensure flatness when used as a current collector. Therefore, by setting the tensile strength of the cladding to a range of 1.20 to 2.40 times the σ value, it is possible to achieve both the necessary strength to maintain the shape of the current collector and the necessary flatness through shape correction of the current collector. From the standpoint of ensuring strength and flatness, the tensile strength of the cladding is preferably 1.30 to 1.80 times the σ value.

[0042] The tensile strength of the Al layer in the annealed state is preferably 20 to 200 MPa, and more preferably 25 to 190 MPa. The tensile strength of the annealed stainless steel layer is preferably 100 to 1200 MPa, and more preferably 120 to 1150 MPa.

[0043] Here, annealing refers to the process of releasing dislocation structures that occur when aluminum or stainless steel is processed into various shapes, such as thin sheets, through heat treatment, recovery, and recrystallization, thereby bringing it to its softest state. For the Al layer, the annealing finish can be reproduced by heat treatment at 350-400°C for 30 minutes or more, which is more than half the melting point of pure Al (approximately 660°C), in the case of a plate shape. On the other hand, the annealed finish on stainless steel layers can be reproduced by heat treatment at 1000-1050°C for 30 minutes or more in the case of sheet metal.

[0044] The tensile strength of each layer is calculated from the Vickers hardness (HV) of each layer after annealing, even after joining them as a cladding. The Vickers hardness is measured using the same method as the Vickers hardness HV on the Al layer surface of the cladding, as described later. The measurement load for the Vickers hardness HV measurement of the stainless steel layer shall be 0.01 kgf (0.098 N) or higher. The conversion from Vickers hardness to tensile strength shall be calculated using the following equation 3. TS=-100+3.73HV...Equation 3 Here, TS [MPa] is the tensile strength.

[0045] The tensile strength of the cladding is preferably over 20 to 1600 MPa, more preferably 30 to 1500 MPa, and even more preferably 35 to 1250 MPa or 35 to 1000 MPa. The tensile strength of the cladding is measured in accordance with JIS Z 2241:2011, using a JIS No. 13B tensile test specimen taken parallel to the rolling direction from the cladding being measured.

[0046] (6) Hardness of the Al layer in the cladding The Al layer can eliminate uneven pressure distribution by undergoing minute elastic or plastic deformation when the battery experiences slight deformation due to expansion and contraction. As a result, the battery's capacity retention improves, leading to an extended battery life. Therefore, in order for the Al layer to reliably undergo minute elastic or plastic deformation, it is preferable that the Vickers hardness HV on the Al layer side surface of the cladding (i.e., the Al layer) be 60 or less. A preferred Vickers hardness HV is 55 or less. On the other hand, if the Al layer is too soft, it may reduce the strength of the cladding. Therefore, although no lower limit is set for the Vickers hardness HV on the Al layer side surface of the cladding (i.e., the Al layer), it is preferable that the HV be 20 or higher.

[0047] Vickers hardness (HV) is measured for the cladding being tested, in accordance with JIS Z 2244:2009 for cladding.

[0048] However, when measuring the Vickers hardness HV, if the depth of the indentation of the indenter for hardness measurement exceeds half of the thickness of the Al layer, the hardness of the stainless steel layer will be strongly affected. Therefore, in order to avoid the depth of the indentation exceeding half of the thickness of the Al layer, based on the value of the thickness of the Al layer and the hardness of the Al layer in the softest state assumed for a clad having an Al layer / stainless steel layer, specifically, a general annealing material of pure aluminum (A1050) (specifically, Vickers hardness HV = 20), the upper limit of the measurement load is determined.

[0049] Specifically, For the Vickers hardness value HV, when the measurement load (test force) is F [N] and the average of the diagonal lengths d1 and d2 [mm] of the indentation is d [mm], the formula: HV = 0.1891×(F / d 2 ) is represented. From this, d can be expressed as d = {0.1891×(F / HV)} 1 / 2 and can be represented as such. When the depth of the indentation is t and the included angle of the regular square pyramid of the indenter (indentation) is ξ (= 136°), since t = d / {2√2×tan(ξ / 2)}, then t = {0.1891×(F / HV)} 1 / 2 ×{2√2×tan(ξ / 2)} -1 = 0.06212×(F / HV) 1 / 2 is represented. Substituting HV = 20 into this formula, the following formula 4 is obtained. t = 0.01389×√F ··· Formula 4

[0050] That is, the measurement load F [N] of the hardness of the Al layer is set to the maximum load that does not exceed 1 / 4 of the thickness of the Al layer among the test forces of the Vickers hardness test, low test force Vickers hardness test, and micro-Vickers hardness test described in JIS Z 2244. For example, if the thickness of the Al layer is 100 μm, it will be 0.03 kgf (0.294 N), and if it is 50 μm, it will be 0.01 gf (0.0981 N).

[0051] (7) Plate thickness ratio of Al layer and stainless steel layer Table 1 shows the thermal conductivity and electrical resistivity of ferritic stainless steel and aluminum (Al). As is clear from Table 1, Al has higher thermal conductivity and lower electrical resistivity compared to ferritic stainless steel. To ensure high electrical and thermal conductivity of the cladding, the thickness ratio of the Al layer must be above a certain level. On the other hand, from the standpoint of ensuring strength, a higher thickness ratio of the stainless steel layer is advantageous. To achieve a good balance between electrical conductivity, thermal conductivity, and strength, the thickness ratio of the Al layer to the stainless steel layer (Al layer / stainless steel layer) is preferably 4 / 6 to 8 / 2. A preferred thickness ratio of Al layer to stainless steel layer is 5 / 5 to 7 / 3. [Table 1]

[0052] The thickness of the stainless steel layer is preferably 5 to 550 μm, and more preferably 10 to 500 μm. The thickness of the Al layer is preferably 2 to 550 μm, more preferably 5 to 520 μm, and even more preferably 15 to 500 μm.

[0053] (8) Cladding plate thickness When the cladding of this disclosure is applied, for example, to a current collector for a large stationary bipolar battery, an in-vehicle bipolar battery, or a bipolar battery for a portable device (such as a smart device), it is preferable that the thickness be up to about 600 μm. Therefore, the cladding thickness is preferably 600 μm or less. A preferred cladding thickness is 580 μm or less. However, from the standpoint of ensuring strength, the lower limit of the cladding plate thickness is preferably 7 μm or more, more preferably 10 μm or more, and even more preferably 20 μm or more.

[0054] (9) Method of manufacturing cladding An example of a method for manufacturing the cladding described herein is explained. The cladding of this disclosure will achieve the above-described effects regardless of the manufacturing method, as long as it possesses the above-described characteristics. However, the manufacturing method of the cladding of this disclosure described below is preferred because it allows for stable production.

[0055] • Dimensions of the material In the cladding manufacturing method of this disclosure, several methods can be considered for joining the materials of each layer, such as rolling bonding, pressure bonding, and explosive bonding, and any of these methods may be used. On the other hand, in order to efficiently manufacture cladding used in the form of plates, foils, etc., such as current collectors, rolling bonding using coil-shaped thin plate material is preferred as the joining method. When manufacturing cladding by the rolling bonding method, the material for each layer is preferably a coiled plate or strip, from a manufacturing standpoint. Due to the difference in strength between the Al layer and the stainless steel layer, the amount of deformation in various rolling processes differs significantly between the Al layer and the stainless steel layer. Specifically, during rolling, the rate of thickness reduction in the stainless steel layer is small, while the rate of thickness reduction in the Al layer is large. Therefore, the thickness ratio of the Al layer material to the stainless steel layer material (thickness of the Al layer material / thickness of the stainless steel layer material) is preferably larger than the thickness ratio of the Al layer to the stainless steel layer in the cladding after manufacturing, for example, preferably 1.7 times or more. More preferably 2.2 times or more.

[0056] When manufacturing cladding by the rolling bonding method, it is preferable to manufacture the cladding by performing the following processes on the material of each of the above layers: a warm bonding rolling process, a cold rolling process, a finish rolling process, and an annealing process. [Warm bonding and rolling process] A process of joining an aluminum layer material and a stainless steel material by heating, lamination, and rolling to form a cladding layer. [Cold rolling process] This process involves cold rolling a clad material, formed by warm bonding and rolling, to adjust its thickness to a level close to the desired thickness. [Finishing Rolling Process] This process involves warm finishing rolling of cold-rolled material to achieve the desired thickness and adjust its tensile strength. [Annealing process] This process involves annealing the finished-rolled material to adjust the tensile strength of the cladding and the strength of the Al layer.

[0057] The following describes each step in order.

[0058] [Warm bonding and rolling process] In the warm joining rolling process, the Al layer material and the stainless steel layer material are heated and then laminated, and the interface is joined by rolling. Since bonding during rolling is promoted by plastic deformation at the interface, it is preferable to perform the process at a warm temperature rather than a cold temperature. Although there is no particular requirement for the heating temperature, it is preferable to raise the material temperature to 150°C or higher before lamination and rolling in order to activate plastic deformation at the interface. From the viewpoint of ensuring bonding strength at the interface, it is more preferable to raise it to 170°C or higher. On the other hand, excessive heating leads to increased energy costs in manufacturing and also to scale formation on the material surface. For this reason, it is preferable to raise the heating temperature to 450°C or lower. From the viewpoint of manufacturing costs, it is more preferable to raise it to 400°C or lower.

[0059] While there are no specific requirements regarding the rolling ratio, it is preferable that it be above a certain level because the plastic deformation of each layer material at the interface enhances the joint strength. Specifically, a rolling ratio of 12% or more is preferable. From the viewpoint of joint strength, a more preferable rolling ratio is 15% or more. On the other hand, if the rolling ratio is excessively high, the load on the equipment becomes unnecessarily large, so a rolling ratio of 35% or less is preferable. From the viewpoint of manufacturing stability, it is more preferable to have a rolling ratio of 32% or less.

[0060] [Cold rolling process] In the cold rolling process, the clad material from the warm joining rolling is brought closer to the desired thickness. The cold rolling process also accumulates strain to sufficiently reduce the tensile strength in the finish rolling process, which will be described later. It is preferable that the rolling ratio in the cold rolling process be 30% or more, and more preferably 40% or more, so that the effect of reducing the tensile strength after rolling is obtained in the finish rolling process. On the other hand, cold rolling at an excessive rolling ratio hardens the material significantly, placing an unnecessarily large load on the equipment and compromising manufacturing stability; therefore, a rolling ratio of 90% or less is preferable. The rolling process can be carried out in one pass or divided into multiple passes.

[0061] [Finishing Rolling Process] In the finishing rolling process, the cold-rolled material is rolled to the desired thickness and simultaneously adjusted to the desired tensile strength. Due to the strain accumulated in the cold rolling process, the warm rolling in the finishing rolling process, and the annealing process described later, the tensile strength of the Al layer and stainless steel layer constituting the cladding becomes 2.4 times or less the value of σ shown in Equation 1. This effect is particularly stable and preferable when the rolling ratio in the finishing rolling process is 10% or more. More preferably, it is 12% or more. On the other hand, if the rolling ratio in the finishing rolling process is excessively high, the load on the equipment becomes unnecessarily large. Therefore, it is preferable that the rolling ratio in the finishing rolling process be 35% or less. From the viewpoint of manufacturing stability, it is more preferable to be 30% or less, and even more preferable to be 25% or less.

[0062] Regarding the rolling temperature, in order to promote softening during rolling, it is preferable to set the rolling rate within the above range and the heating temperature during rolling to 420°C or higher. From the viewpoint of obtaining a stable softening effect, it is more preferable to set it to 445°C or higher. On the other hand, if cold-rolled material is excessively heated, Fe-Al intermetallic compounds may form at the metal layer interface, potentially causing interfacial delamination. Therefore, a rolling temperature of 550°C or lower is preferable.

[0063] [Annealing process] In the annealing process, the finished-rolled material is annealed to adjust the tensile strength of the cladding and the strength of the Al layer. Annealing primarily softens the Al layer. The annealing temperature is preferably 200°C or higher. More preferably 225°C or higher, from the viewpoint of ensuring the softening effect of the Al layer. On the other hand, if the finished-rolled material is excessively heated, Fe-Al intermetallic compounds may be formed at the metal layer interface, potentially causing interfacial delamination, and the excessive heating is uneconomical from the viewpoint of energy costs. For this reason, the annealing temperature is preferably 550°C or lower. Considering manufacturing costs, it is more preferably 520°C or lower, and even more preferably 500°C or lower. While there is no specific requirement for the annealing holding time, it is preferable to hold it for 5 minutes or more, as this ensures a stable softening effect on the Al layer. More preferably, it is 10 minutes or more. There is also no upper limit for the annealing holding time, but from the viewpoint of energy costs associated with the heat treatment, it is preferable to keep it to 100 hours or less.

[0064] In the cladding manufacturing method described above, cladding with a tensile strength of 1.20 to 2.40 times the σ value shown in Equation 1 can be stably produced, particularly depending on the conditions of each manufacturing process, from the cold rolling process to the finish rolling process and the heat treatment process. The exact cause is not entirely clear, but the following reasons are possible. Specifically, by introducing dislocations into the stainless steel layer at a certain high density range during the cold rolling process, and then applying warm rolling, the atoms in the stainless steel layer self-diffuse at a certain rate due to the action of the dislocations and the rolling process. This may result in a phenomenon where the reconstruction of the crystal structure is promoted even in temperature ranges where recovery or recrystallization does not normally occur, thus stably achieving a tensile strength in the range of 1.20 to 2.40 times the σ value shown in Equation 1.

[0065] <Current collector for bipolar battery> The current collector of the bipolar battery of this disclosure consists of the cladding of this disclosure. Specifically, for example, the current collector of the bipolar battery of the present disclosure is made of a cut material of the cladding of the present disclosure.

[0066] In this case, of the two opposing surfaces in the thickness direction of the current collector of a bipolar battery, the positive electrode is provided on the Al layer surface and the negative electrode is provided on the stainless steel layer surface. In a bipolar battery, different electrode reactions occur at the positive and negative electrodes. As shown in Figure 1, if burrs (i.e., flash) from the Al layer generated at the cut ends of the cladding that constitutes the current collector wrap around to the surface of the stainless steel layer, they may inhibit the electrode reaction of the negative electrode on the surface of the stainless steel layer. Specifically, this is as follows:

[0067] The cladding of the current collector according to this disclosure has reduced ductility in the stainless steel layer due to the introduction of dislocations, and is subjected to slit cutting, for example. Therefore, when the slitting blade is inserted from the hard stainless steel layer side, the warping of the stainless steel layer that occurs during cutting is not ductile, and it breaks at the cutting point and detaches from the end face, making it difficult to leave a burr. On the other hand, if the slitting blade is inserted from the softer Al layer side, the burr of the Al layer may collapse towards the stainless steel layer. While Al is stable within the potential range in which it is used as a positive electrode current collector, if the burr of the Al layer collapses onto the surface of the negative electrode current collector during slitting (i.e., if the direction of the burr at the cut part of the Al layer is from the interface between the Al layer and the stainless steel layer towards the stainless steel layer), the Al is exposed to the operating potential environment of the negative electrode current collector, and an Al-Li alloy is formed. Furthermore, repeated use as a battery leads to repeated formation of the Al-Li alloy and desorption of Li from the alloy, causing repeated volume expansion and contraction at that location, which can cause the Al to pulverize and lead to a short circuit in the battery.

[0068] For this reason, it is preferable to insert the slitting blade from the stainless steel layer side and cut so that, for example, the direction of the burr of the cut portion of the Al layer is directed toward the Al layer from the bonding interface between the Al layer and the stainless steel layer. In other words, as shown in Figure 2, it is preferable that the end of the current collector has a return of the Al layer in the direction of the Al layer from the bonding interface between the Al layer and the stainless steel layer. In Figures 1 and 2, 10 represents the cladding, 12 represents the stainless steel layer, 14 represents the Al layer, and 14A represents the return layer of the Al layer.

[0069] <Bipolar battery> The bipolar battery of this disclosure has a current collector of the bipolar battery of this disclosure. Specifically, the bipolar battery of this disclosure, as shown in Figure 3, for example, has a plurality of bipolar electrodes and a plurality of solid electrolyte layers, and the bipolar electrodes are repeatedly stacked via the solid electrolyte layers. The bipolar electrode comprises the cladding described above, a positive electrode provided on the Al layer surface of the cladding, and a negative electrode provided on the stainless steel layer surface of the cladding. In the bipolar battery of this disclosure, the positive electrode, negative electrode, and solid electrolyte layer employ well-known configurations. In Figure 3, 10 represents the cladding, 12 the stainless steel layer, 14 the Al layer, 16 the positive electrode, 18 the negative electrode, 20 the bipolar electrode, and 22 the solid electrolyte layer. [Examples]

[0070] The following describes examples of tests conducted to demonstrate the effectiveness of this disclosure, but this disclosure is not limited to the following examples.

[0071] (1) Test material Table 2 shows the Al layer materials used in the test. Codes A1 and A2 are in the 1000 series as defined in JIS H 4000:2014, and represent industrial-grade pure Al with an Al purity of 99% or higher. On the other hand, codes A3 and A4 are alloys with an Al purity of less than 99%, corresponding to the 3000 and 5000 series, respectively, as defined in JIS H 4000:2014. All Al layer materials used were annealed. The tensile strength was calculated using Equation 3, based on the Vickers hardness values ​​measured according to the method compliant with JIS Z 2244:2009.

[0072] Table 3 shows the stainless steel layer materials used in the test. Each stainless steel material was as follows: It was melted as a 150 kg ingot in a vacuum induction melting furnace, and then hot-rolled at 1250°C to a hot-rolled sheet with a thickness of 4 mm. The obtained hot-rolled sheet was then pickled and cold-rolled to make a cold-rolled sheet, and then finished annealing and pickling were performed to obtain a material in an annealed finish state. The tensile strength was calculated using Equation 3 from the Vickers hardness value measured in accordance with JIS Z 2244:2009, the same as for the Al material.

[0073] (2) Manufacturing of cladding Using Al layer materials A1-A4 and stainless steel layer materials S1-S5, warm joining rolling was performed at a heating temperature of 300°C and a rolling rate of 20%. This was followed by cold rolling at various rolling rates, and then finish rolling at various temperatures and rolling rate conditions. The finished thickness for all samples was adjusted to 500 μm by controlling the material thickness, etc. Furthermore, some of the finished-rolled samples were annealed at various temperatures for 1 hour. The rolling ratio for cold rolling, the rolling ratio for finish rolling, the heating temperature, and the annealing conditions are shown in Table 4.

[0074] (3) Single material To compare the performance of Al and stainless steel as current collectors in bipolar batteries with that of cladding, the individual materials listed in Tables 2 and 3 were subjected to cold rolling, finish rolling, and annealing to obtain individual Al and stainless steel materials.

[0075] (4) Evaluation The following properties were measured for the fabricated clad and individual materials according to the methods described above. However, Vickers hardness (HV) measurements were not performed for the individual materials. • Tensile strength of clad and solid materials (indicated as "Tensile strength of clad" in the table) • Tensile strength of the Al layer in an annealed state • Tensile strength of the stainless steel layer in an annealed state • Clad and solid material thickness (indicated as "Clad Thickness" in the table) • σ value shown in Equation 1 • Vickers hardness HV on the Al layer side surface of the cladding (indicated as "hardness of the Al layer")

[0076] Next, the fabricated cladding and individual materials were each incorporated as current collectors into simulated cells for laboratory evaluation, and the following evaluations were conducted. Specifically, the evaluations were as follows:

[0077] A bipolar electrode was fabricated by forming a positive electrode on the Al layer surface and a negative electrode on the stainless steel layer surface of the prepared cladding. Then, in an Ar-atmosphere glove box, the fabricated bipolar electrodes and solid electrolyte layers were alternately stacked. The resulting stack of three bipolar electrodes and two solid electrolyte layers was then placed inside a simulated cell device (φ10 mm cylindrical cell) for laboratory evaluation and subjected to pressure restraint. This created an evaluation bipolar battery that could be evaluated while the internal stack was under pressure, as shown in Figure 4. Furthermore, evaluation bipolar batteries were fabricated in the same manner, except that a solid material was used instead of a cladding layer. Furthermore, the laminate was constructed such that both sides were composed of current collectors. In Figure 4, 10 represents the cladding, 12 the stainless steel layer, 14 the Al layer, 16 the positive electrode, 18 the negative electrode, 20 the bipolar electrode, and 22 the solid electrolyte layer.

[0078] Here, the solid electrolyte layer is Li7La3Zr2O 12 Li Limited 0.5 Mn 1.5 Formed using O4. The positive electrode was formed as follows: A positive electrode material with the same material composition as the solid electrolyte layer, and a solid electrolyte in powder form with an average particle size of 15 μm or less (Li7La3Zr2O) to ensure a Li ion conduction path. 12 A mixture of acetylene black powder with an average particle size of 0.1 μm to ensure conductive paths was prepared, and the mixture was slurryed with a PVDF (PolyVinylidene DiFluoride) binder dissolved in an NMP (N-methylpyrrolidone) solution to obtain a coating solution. The obtained coating solution was applied to the Al layer surface of the cladding and one surface of the individual material, dried, and then pressurized to form a positive electrode. The negative electrode was formed using Li metal on the Al layer surface of the cladding and on one side of the individual material.

[0079] The obtained bipolar batteries were evaluated for their 100-cycle capacity retention rate and maximum battery temperature. Specifically, the results are as follows: Battery capacity retention was evaluated by repeatedly charging and discharging evaluation bipolar batteries placed in a 45°C environment. The discharge capacity at 100 charge / discharge cycles was divided by the maximum discharge capacity, and the value was calculated in percentage. A low value indicates a significant degree of battery performance degradation. Note that some bipolar batteries using certain current collectors became unable to charge or discharge at some point; the results are shown in Table 4. In terms of battery capacity retention, less than 40% was judged as unacceptable, 40-80% as acceptable, and 80% or more as good. The charging method is constant current constant voltage (CCCV), 0.1mA / cm². 2 The test was conducted under the condition that the voltage was maintained at a constant level for 2 hours after reaching 10V. Discharge was performed using a constant current (CC) method, at 0.1mA / cm². 2 The test terminated at 5.2V. The repetition was performed without any intervals.

[0080] The maximum battery temperature was measured using a thermocouple on the side of the battery, and the highest temperature recorded during the test was recorded. A maximum battery temperature of 80°C or higher was considered unacceptable, 60°C or higher was acceptable, and below 60°C was considered good.

[0081] (5) Evaluation results The results of the above test are shown in Table 4. Tests 1-4 were conducted to investigate the effect of the Cr content in the stainless steel material constituting the cladding of the disclosed material. The capacity retention rate of the all-solid-state battery, evaluated under identical conditions for Al type and cladding thickness ratio, was low at 32% in Test 1, where the Cr content was lower than that within the scope of the disclosed material. Corrosion was observed in the stainless steel layer after battery evaluation, which was thought to have led to a significant decrease in capacity retention rate. In Test 4, the stainless steel layer had a high Cr content, and in addition to the battery becoming hotter, the capacity retention rate fell below 40%. Tests 5 and 30 were conducted to investigate the effects of Mo. In Test 5, the Mo content was high, and due to Joule heating of the stainless steel layer, the battery temperature exceeded 60°C, similar to Test 4. Even without Mo, Test 30 exhibits a tensile strength 1.20 to 2.40 times σ calculated using the combined law of Equation 1, and the battery capacity retention rate is good at over 80%.

[0082] Tests 10, 13, 18, 26, and 28 use the same cladding material, but the cladding strength differs. Tests 10, 26, and 28 showed that the tensile strength of the clad material was in the range of 1.20 to 2.40 times σ calculated using the composite law of Equation 1, and the battery capacity retention rate was good at over 80%. On the other hand, in Test 13, the tensile strength was lower than 1.2 times σ, and the capacity retention rate was below 80%. In the cell incorporating the cladding in Test 13, contraction occurred in the cell structure while under pressure. This is thought to be due to deformation of some components due to insufficient strength of the current collector. As a result, the stress on the electrodes was relaxed, making it impossible to maintain proper pressure, and thus the battery's capacity was not fully realized. Furthermore, in test 18, the tensile strength was higher than 2.4 times σ, and the volume retention rate was below 70%. This is thought to be because the high strength of the cladding prevented sufficient shape correction during the manufacturing of the cladding sample, resulting in uneven pressure conditions during the test.

[0083] Tests 3, 6, 7, and 8 investigated the effect of the purity of the Al material constituting the cladding. In tests 3 and 6, where pure Al with a purity of 99% or more within the scope of this disclosure was used as the constituent material for the Al layer, the capacity retention rate was 70% or higher. On the other hand, in tests 7 and 8, where Al alloys A3 and A4 were used as the constituent material for the Al layer, the capacity retention rate was below 70%. This is thought to be due to the dissolution of the alloy components of the Al alloy material in the Al layer due to corrosion of the Al layer.

[0084] Test 12 shows the results when the direction of slitting was intentionally changed from the Al layer to the stainless steel layer, causing the Al burr at the cut point to bend from the cladding interface towards the stainless steel layer. In this case, the battery temperature exceeded 60°C, and the capacity retention rate fell below 70%. As mentioned earlier, this is presumed to be because the Al was in contact with the Li of the negative electrode, causing the Al to pulverize.

[0085] Tests 2, 9, 10, 11, 20, 21, 22, and 27 investigated the effect of the thickness ratio of the Al layer and stainless steel layer constituting the cladding. As in Test 11, when the thickness ratio of the Al layer increases beyond the thickness ratio range of this disclosure, the strength of the cladding material decreases. This is thought to be because the amount of work hardening of Al is smaller compared to stainless steel. As a result, in Test 11, the capacity retention rate fell below 70%. This is presumed to be due to insufficient strength of the current collector, making it difficult to maintain the appropriate pressure, similar to Test 13.

[0086] Tests 14 and 15 show the results when stainless steel alone was used as the current collector material, while Test 16 shows the results when aluminum alone was used. In Tests 14 and 15, the battery temperature exceeded 60°C, and the capacity retention rate decreased significantly. In Test 16, the battery capacity rapidly decreased immediately after the start of the test, and the battery ceased to function after 5 cycles. Upon disassembly and investigation of the cell in Test 16, it was found that the aluminum in the current collector had alloyed with the lithium in the negative electrode, causing cracks to appear on the surface. Tests 2 and 17 investigated the effect of the hardness of the Al layer constituting the cladding. In Test 17, the capacity retention rate at 100 cycles was slightly below 70%. This is thought to be because the hardness of the Al layer caused uneven pressure due to the expansion and contraction of the battery.

[0087] In Test 19, the tensile strength was lower than 1.2 times σ due to a low rolling ratio during the finishing roll, and the capacity retention rate was below 80%. Furthermore, the battery temperature exceeded 60°C. In Test 23, the tensile strength is higher than 2.4 times σ due to the low rolling temperature during the finish rolling process, and the capacity retention rate is below 80%. In tests 24 and 25, the finishing rolling temperature or annealing temperature was high, causing delamination during the processing of the test specimens. In test 29, the capacity retention rate was below 80% because an austenitic stainless steel layer was used.

[0088] [Table 2]

[0089] [Table 3]

[0090] [Table 4]

[0091] From the above results, the cladding of this disclosure is a cladding that possesses corrosion resistance, electrical conductivity and thermal conductivity, while ensuring strength for shape maintenance and flatness through shape correction, thereby suppressing degradation of battery performance. Because the cladding of this disclosure achieves high corrosion resistance, strength, electrical conductivity and thermal conductivity simultaneously, it is a material suitable not only for current collectors of bipolar electrodes but also for heat sinks and the like. [Explanation of Symbols]

[0092] 10 Clad 12 stainless steel layers 14 Al layer 14A Al layer return 16 positive electrode 18 negative electrode 20 bipolar electrodes 22 Solid electrolyte layer

Claims

1. It has an Al layer and a stainless steel layer bonded to the Al layer, The aforementioned Al layer is a pure Al layer with an Al content of 99% or more by mass. The aforementioned stainless steel layer is a ferritic stainless steel layer containing, by mass%, Cr: 13.0-19.0% and Mo: 0-0.5%. The tensile strength of the cladding is between 1.20 and 2.40 times the σ value shown in equation 1 below. The Vickers hardness HV on the Al layer side surface of the cladding is 60 or less. A cladding material with a plate thickness of 600 μm or less. σ=α×x+β(1-x) ...Formula 1 In Equation 1, α, β, and x represent the tensile strength of the annealed Al layer, the tensile strength of the annealed stainless steel layer, and the ratio of the thickness of the Al layer to the thickness of the cladding, respectively.

2. The cladding according to claim 1, wherein the thickness ratio of the Al layer to the stainless steel layer (Al layer / stainless steel layer) is 4 / 6 to 8 / 2.

3. A cladding according to claim 1 or claim 2 for a current collector of a bipolar battery.

4. A current collector for a bipolar battery comprising the cladding according to any one of claims 1 to 3.

5. The current collector for a bipolar battery according to claim 4, wherein at the end of the current collector, there is a return of the Al layer in the direction of the Al layer from the bonding interface between the Al layer and the stainless steel layer.

6. A bipolar battery having a current collector according to claim 4 or claim 5.

7. It has an Al layer and a stainless steel layer bonded to the Al layer, The aforementioned Al layer is a pure Al layer with an Al content of 99% or more by mass. The aforementioned stainless steel layer is a ferritic stainless steel layer containing, by mass%, Cr: 13.0-19.0% and Mo: 0-0.5%. The tensile strength of the cladding is between 1.20 and 2.40 times the σ value shown in equation 1 below. Cladding with a plate thickness of 600 μm or less σ=α×x+β(1-x) ...Formula 1 In Equation 1, α, β, and x represent the tensile strength of the annealed Al layer, the tensile strength of the annealed stainless steel layer, and the ratio of the thickness of the Al layer to the thickness of the cladding, respectively.

8. A cladding comprising an Al layer and a stainless steel layer bonded to the Al layer, The aforementioned Al layer is a pure Al layer with an Al content of 99% or more by mass. The aforementioned stainless steel layer is a ferritic stainless steel layer containing, by mass%, Cr: 13.0-19.0% and Mo: 0-0.5%. The tensile strength of the cladding is between 1.20 and 2.40 times the σ value shown in equation 1 below. Cladding for current collectors in bipolar batteries. σ=α×x+β(1-x) ...Formula 1 In Equation 1, α, β, and x represent the tensile strength of the annealed Al layer, the tensile strength of the annealed stainless steel layer, and the ratio of the thickness of the Al layer to the thickness of the cladding, respectively.

9. A current collector for a bipolar battery, comprising a cladding having an Al layer and a stainless steel layer bonded to the Al layer, The aforementioned Al layer is a pure Al layer with an Al content of 99% or more by mass. The aforementioned stainless steel layer is a ferritic stainless steel layer containing, by mass%, Cr: 13.0-19.0% and Mo: 0-0.5%. A current collector for a bipolar battery, wherein the tensile strength of the cladding is 1.20 to 2.40 times the σ value shown in equation 1 below.

10. A current collector for a bipolar battery comprising the cladding described in claim 7 or claim 8.

11. A bipolar battery having a current collector according to claim 9 or claim 10.

Images