Battery case for lithium ion batteries and lithium ion battery

Abstract

The present invention addresses the problem of achieving excellent safety and electrolyte resistance. Provided as a solution is a battery case for lithium ion batteries comprising: a case body section which accommodates a battery unit that has a positive electrode, a negative electrode, and a separator, and an electrolyte solution that contains a lithium salt; and a lid section which seals the case body section, wherein the material of the case body section and the lid section is a steel material having a chemical composition containing, in terms of mass%, 1.0-9.9% Cr, 0.5-10.0% Al, 0.1-5.0% Mn, 0.002-0.100% C, 0.01-0.50% Si, 0.005-0.070% P, 0.001-0.250% S, 0.001-0.020% N, and 0.0001-0.0030% B, with the remainder being Fe and impurities. The battery case has a welded part where the case body section and the lid section are welded.

Description

Battery case and lithium-ion battery for lithium-ion batteries

[0001] This invention relates to a battery case for lithium-ion batteries and a lithium-ion battery.

[0002] In recent years, the applications of lithium-ion batteries have been rapidly expanding to include energy storage devices for power generation combined with new energy systems such as solar cells and wind power, as well as automotive batteries. When considering applications as automotive batteries, the material of the lithium-ion battery case must be both lightweight and corrosion-resistant.

[0003] The corrosion resistance required for a lithium-ion battery case includes not only the corrosion resistance of the outer surface of the battery case (the side exposed to the atmosphere, etc.) (hereinafter referred to as "external corrosion resistance"), but also the corrosion resistance of the electrolyte containing lithium salts (e.g., LiPF) contained inside the battery case. ６ Corrosion resistance to (and other similar substances) (hereinafter referred to as "electrolyte resistance") is taken into consideration. For this reason, aluminum is often used as the material for lithium-ion battery cases because it allows for the battery case itself to be lightweight and also possesses the two types of corrosion resistance mentioned above.

[0004] Japanese Patent Publication No. 2001-164336

[0005] In the automotive lithium-ion battery market, there is a demand for increased capacity and energy density to improve the performance of electric vehicles, leading to a trend towards larger lithium-ion batteries (increasing the battery's volume). However, larger battery volumes tend to trap more heat. Therefore, if the lithium-ion battery's temperature rises excessively and leads to thermal runaway, the battery case could melt, exceeding the melting point of aluminum (approximately 660°C). Thus, as lithium-ion batteries become more widespread in automobiles, greater safety of the battery itself is essential.

[0006] From a safety perspective, as described above, it is conceivable to use a material with a higher melting point than aluminum to prevent the battery case from melting even if the lithium-ion battery experiences thermal runaway. One such material is steel. The melting point of steel is generally around 1500°C, although this depends on its chemical composition. Therefore, by using steel as the material for the battery case, it is possible to prevent the battery case from melting during thermal runaway, even if the lithium-ion battery itself is made larger.

[0007] On the other hand, regarding the corrosion resistance of lithium-ion battery cases, steel has conventionally been used as a material for various products and structures used in a variety of corrosive environments, such as high-temperature humid corrosion environments, atmospheric corrosion environments, condensation corrosion environments, tap water corrosion environments, drinking water corrosion environments, concrete corrosion environments, and seawater corrosion environments. Furthermore, various technologies have been proposed to realize steel materials with excellent corrosion resistance.

[0008] For example, Patent Document 1 proposes a steel material that exhibits excellent corrosion resistance of processed parts even under various corrosive environments as described above. This steel material uses a steel containing Cr, Al, and Mg at specific concentrations as a base material, and has a metal layer that satisfies specific electrochemical conditions on the surface of the base material.

[0009] Considering the battery case of a lithium-ion battery, the inside of the lithium-ion battery contains a highly reactive compound called a lithium salt containing fluorine as the electrolyte. Furthermore, the environment inside the lithium-ion battery is an extremely unique corrosive environment, as it is exposed to both a strong oxidizing atmosphere and a strong reducing atmosphere within the confined space of the battery case. From the above perspective, even with steel materials intended to improve corrosion resistance under various corrosive environments, such as those described in Patent Document 1, there was still room for further improvement in terms of electrolyte resistance of the battery case in a lithium-ion battery.

[0010] Thus, when focusing on the battery case of a lithium-ion battery, there is currently no material that combines the safety required of a lithium-ion battery with the electrolyte resistance required for a battery case. Therefore, there is a strong demand for a battery case for lithium-ion batteries that offers superior safety and electrolyte resistance.

[0011] Therefore, the present invention has been made in view of the above problems, and the object of the present invention is to provide a battery case for lithium-ion batteries and a lithium-ion battery that can achieve better safety and electrolyte resistance.

[0012] To solve the above problems, the inventors conducted diligent research and found that by using steel as the material for the battery case of a lithium-ion battery, which offers superior safety, it is possible to achieve better electrolyte resistance by keeping the chemical composition of the steel within a specific range. Based on this finding, the gist of the present invention is as follows.

[0013] (1) A battery case for a lithium-ion battery, comprising: (1) a battery unit having a positive electrode, a negative electrode and a separator; an electrolyte containing a lithium salt; a case body portion for housing the battery unit; and a lid portion for sealing the case body portion, wherein the material of the case body portion and the lid portion is steel having a chemical composition in mass percent of Cr: 1.0 to 9.9%, Al: 0.5 to 10.0%, Mn: 0.1 to 5.0%, C: 0.002 to 0.100%, Si: 0.01 to 0.50%, P: 0.005 to 0.070%, S: 0.001 to 0.250%, N: 0.001 to 0.020%, B: 0.0001 to 0.0030%, with the remainder being Fe and impurities, and the battery case has a welded joint where the case body portion and the lid portion are welded together. (2) A battery case for a lithium-ion battery, comprising a battery unit having a positive electrode, a negative electrode and a separator, an electrolyte containing a lithium salt, a case body for housing the battery unit, and a lid for sealing the case body, wherein the material of the case body and the lid is, in mass%, Cr: 1.0 to 9.9%, Al: 0.5 to 10.0%, Mn: 0.1 to 5.0%, C: 0.002 to 0.100%, Si: 0.01 to 0.50%, P: A battery case for a lithium-ion battery, comprising steel having a chemical composition containing 0.005 to 0.070% of solids, 0.001 to 0.250% of sulfur, 0.001 to 0.020% of nitrogen, and 0.0001 to 0.0030% of nitrile, and further containing one or more elements selected from the group consisting of elements A to D below, with the remainder being Fe and impurities, wherein the battery case has a welded joint where the case body and the lid are welded together.(3) The battery case for a lithium-ion battery according to (1) or (2), wherein the welded part includes a weld metal composed of an alloy of the chemical components contained in the material of the case body and the lid. (4) A battery case for a lithium-ion battery according to (1) or (2), wherein at least one of Cr fluoride or Al fluoride is present on the inner surface of the battery case in contact with the electrolyte. (5) A battery case for a lithium-ion battery according to (1) or (2), wherein in the welded portion, an oxide film layer is present on the inner surface of the battery case in a portion that may come into contact with the electrolyte. (6) A battery case for a lithium-ion battery according to (5), wherein the thickness of the oxide film layer is 0.01 to 1.00 μm. (7) A battery case for a lithium-ion battery according to (6), wherein the thickness of the oxide film layer is 0.01 to 0.50 μm. (8) A battery case for a lithium-ion battery according to (1) or (2), wherein the thickness of the case body is 0.10 to 1.00 mm, and the thickness of the lid is 0.10 to 1.00 mm. (9) The battery case for a lithium-ion battery according to (1) or (2), wherein the steel material is a steel material without a plating layer. (10) The battery case for a lithium-ion battery according to (1) or (2), wherein the welded part is a laser welded part. (11) The battery case for a lithium-ion battery according to (2), having a chemical composition containing the element group A. (12) The battery case for a lithium-ion battery according to (2), having a chemical composition containing the element group B. (13) The battery case for a lithium-ion battery according to (2), having a chemical composition containing the element group C. (14) The battery case for a lithium-ion battery according to (2), having a chemical composition containing the element group D.(15) A lithium-ion battery having a battery case for a lithium-ion battery as described in (1) or (2).

[0014] As described above, the present invention makes it possible to provide a battery case for lithium-ion batteries and a lithium-ion battery that have superior safety and electrolyte resistance.

[0015] This is a schematic explanatory diagram showing a battery case for a lithium-ion battery according to an embodiment of the present invention, and a lithium-ion battery using such a battery case. This is a schematic explanatory diagram showing a battery case for a lithium-ion battery according to the same embodiment, and a lithium-ion battery using such a battery case. This is a schematic explanatory diagram showing a battery case for a lithium-ion battery according to the same embodiment, and a lithium-ion battery using such a battery case. This is a schematic diagram for explaining the structure near the weld of the battery case for a lithium-ion battery according to the same embodiment. This is a schematic diagram for explaining the structure near the weld of the battery case for a lithium-ion battery according to the same embodiment. This is a schematic diagram for explaining the structure on the inner surface of the battery case near the weld of the battery case for a lithium-ion battery according to the same embodiment. This is a schematic diagram for explaining the structure on the inner surface of the battery case near the weld of the battery case for a lithium-ion battery according to the same embodiment. This is a schematic explanatory diagram showing the material of the battery case for a lithium-ion battery according to the same embodiment.

[0016] Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. In this specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant explanations will be omitted. In the following description, the notation "numerical value A to numerical value B" indicates that the values ​​are greater than or equal to numerical value A and less than or equal to numerical value B.

[0017] (Regarding Lithium-ion Batteries) <Overall Configuration of Lithium-ion Batteries> First, the overall configuration of the lithium-ion battery according to the embodiment of the present invention will be described with reference to Figures 1 to 2B. Figures 1 to 2B are schematic explanatory diagrams showing a battery case for a lithium-ion battery according to this embodiment and a lithium-ion battery using such a battery case. For convenience, the coordinate system shown in Figure 1 may be used in the following explanation.

[0018] As schematically shown in Figure 1, the lithium-ion battery 1 according to this embodiment comprises a battery unit 3 having a positive electrode, a negative electrode, and a separator, and an electrolyte 5 containing a lithium salt, all housed in a battery case 10 for the lithium-ion battery according to this embodiment (hereinafter simply abbreviated as "battery case 10"). Here, the battery case 10 according to this embodiment has a case body portion 11 and a lid portion 13.

[0019] [Regarding the battery unit 3 and electrolyte 5] Here, the positive electrode (not shown), negative electrode (not shown), separator (not shown), and various active materials (not shown) provided on the positive and negative electrodes that constitute the battery unit 3 are not particularly limited. Various types used in lithium-ion batteries can be used as appropriate for each component constituting the battery unit 3. Furthermore, the specific structure of the battery unit 3 is not particularly limited, and various structures can be adopted.

[0020] Furthermore, the specific shape and size of the battery unit 3 are not particularly limited. In Figure 1, a battery unit 3 having a rectangular shape is shown for convenience, but the shape of the battery unit 3 may be cylindrical or any other shape.

[0021] Furthermore, the electrolyte 5 only needs to contain a lithium salt capable of generating lithium ions, and various electrolytes containing lithium salts (especially lithium salts containing fluorine) can be used as appropriate. For example, LiPF ６ LiBF ４ , LiN (SO２ CF ３ ) ２ One example is (also known as LiTFSI).

[0022] [Regarding the battery case 10] As shown in Figure 1, the battery case 10 in the lithium-ion battery 1 according to this embodiment houses the battery unit 3 and electrolyte 5 as described above, and is composed of a case body 11 and a lid 13.

[0023] The case body 11 of the battery case 10 is a hollow member having an internal space capable of housing the battery unit 3 and the electrolyte 5. The case body 11 is composed of a bottom surface (not shown) and side surfaces. In Figure 1, the case body 11 has a rectangular bottom surface (not shown) and four side surfaces that create an internal space for housing the battery unit 3 and the electrolyte 5. After the battery unit 3 and the electrolyte 5 are housed in the internal space of the case body 11, the opening of the case body 11 is closed by the lid 13, as shown in Figure 1.

[0024] In Figure 1, the case body 11 of the battery case 10 is shown to have a rectangular shape. However, the specific shape of the case body 11 is not particularly defined. The case body 11 can have any shape as long as it is capable of accommodating the battery unit 3 and the electrolyte 5. A case body 11 having such a shape can be manufactured, for example, by deep drawing of the steel material.

[0025] Furthermore, the lid portion 13 is provided with an injection port 15, which is an opening for injecting the electrolyte into the storage case, and after the electrolyte is injected, the injection port 15 is closed with the injection port cover 17.

[0026] In the battery case 10 shown in Figure 1, after the battery unit 3 is housed in the internal space of the case body 11, the opening of the case body 11 is closed by the lid 13 and sealed by welding. As a result, the case body 11 and the lid 13 of the battery case 10 are integrated. Subsequently, electrolyte 5 is injected into the inside of the battery case 10 through the liquid injection port 15 provided in the lid 13. After the electrolyte 5 is injected, the liquid injection port 15 provided in the lid 13 is closed by the liquid injection port cover 17 and sealed by welding. As a result, as schematically shown in Figure 2A, welded parts 21 are formed at the welded parts between the body 11 and the lid 13, and between the lid 13 and the liquid injection port cover 17.

[0027] Here, various known welding methods can be used for welding the case body portion 11 and the lid portion 13 of the battery case 10, and for welding the lid portion 13 and the liquid filling port cover 17. One such welding method is laser welding.

[0028] Furthermore, in Figures 1 to 2A, for illustrative purposes, the case body portion 11 of the battery case 10 is depicted as being composed of a single component. However, the case body portion 11 may be composed of a single component, or it may be composed of multiple components joined together by various welding methods (for example, seam welding or laser welding).

[0029] For example, when forming the side surface of a case body 11 having a rectangular shape as shown in Figure 1, two members are prepared by bending a steel plate, which will be the material for the battery case 10, into a roughly U-shape. These roughly U-shaped members are then butted together, overlapping some of their ends as needed, to form the shape of the side surface of the case body 11. The butt joints are then welded together by overlap seam welding or laser welding to form a joint. Alternatively, the steel plate, which will be the material for the battery case, is processed into a roughly cylindrical shape by bending it four times. The ends of the steel plates are then butted together, overlapping some of them as needed, to form the shape of the side surface of the case body 11. The butt joints are then welded together by overlap seam welding or laser welding to form a joint. After that, a steel material that will form the bottom surface can be welded to the side surface of the case body 11 obtained in the manner described above by laser welding or the like. In this case, the steel material that will form the bottom surface should be shaped to match the shape of the body that will form the side surface of the case body 11. When the case body 11 is manufactured in this manner, the resulting battery case 10 will have welded joints 21 formed on the sides and bottom of the case body 11, as schematically shown in Figure 2B.

[0030] Furthermore, instead of welding as described above, a closure method using blind rivets or the like may be used to close the injection port 15 with the injection port cover 17.

[0031] Figures 3A and 3B are schematic diagrams illustrating the structure near the weld of the battery case for a lithium-ion battery according to this embodiment. Figures 4A and 4B are schematic diagrams illustrating the structure on the inner surface of the battery case near the weld of the battery case for a lithium-ion battery according to this embodiment. Figures 3A to 4B focus on the cross-section when the battery case 10 is cut in the height direction of the battery case. Note that the mechanism for preventing the lid 13 from falling into the internal space of the case body 11 is not shown in Figures 3A to 4B.

[0032] As mentioned above, in the lithium ion battery 1, after the battery unit 3 is housed in the internal space of the case main body 11 of the battery case 10, the lid 13 is arranged to seal the opening of the case main body 11, and the case main body 11 and the lid 13 are welded. Then, the electrolytic solution 5 is injected from the liquid injection port 15 provided in the lid 13, the liquid injection port 15 is closed by the liquid injection port lid 17, and the lid 13 and the liquid injection port lid 16 are welded. As a result, a welded part 21 is formed in the battery case 10 of the lithium ion battery 1.

[0033] Here, in the battery case 10, the position where the welded part 21 is formed is not particularly limited because it depends on the structures of the case main body 11 and the lid 13. However, although it also depends on the structures of the case main body 11 and the lid 13, as shown in FIG. 3A, the welded part 21 may be formed so as to cover the end face of the case main body 11 and the end face of the lid 13, or as shown in FIG. 3B, the welded part 21 is often formed so as to cover a part of the side surface of the case main body 11 and the end face of the lid 13.

[0034] On at least a part of the surface of the welded part 21 formed in this way (the surface on the side in contact with the atmosphere), an oxide film layer (not shown) is formed. By forming an oxide film layer on the surface of the welded part 21, it is possible to prevent the corrosion factors in the atmosphere from directly contacting the welded part 21, and the corrosion resistance of the welded part 21 can be improved.

[0035] Here, as shown in FIGS. 3A and 3B, there may be a case where welding is performed after the case main body 11 and the lid 13 are in contact without a gap and the welded part 21 is formed. However, in some cases, as schematically shown in FIGS. 4A and 4B, there may also be a case where a gap exists between the case main body 11 and the lid 13. As schematically shown in FIGS. 4A and 4B, when a gap exists between the case main body 11 and the lid 13, it is conceivable that the electrolytic solution 5 penetrates into this gap and the surface of the welded part 21 on the inner surface side of the battery case comes into contact with the electrolytic solution 5.

[0036] However, as mentioned above, an oxide film layer is formed on the surface of the welded portion 21 due to welding. Such an oxide film layer is also formed on the surface of the welded portion 21 on the inner surface side of the battery case. By covering the surface of the welded portion (particularly, the weld metal), such an oxide film layer can prevent direct contact between the electrolytic solution 5 and the weld metal, and contribute to the electrolytic solution resistance of the welded portion 21.

[0037] [Regarding the weld metal 201] As schematically shown in FIGS. 4A and 4B, the welded portion 21 according to the present embodiment has a weld metal 201. Further, in the portion of the case main body portion 11 on the side of the weld metal 201 and the portion of the lid portion 13 on the side of the weld metal 201, a heat-affected zone (HAZ) 203 exists due to the heat generated during the welding of the case main body portion 11 and the lid portion 13. Such a weld metal 201 and heat-affected zone 203 can be easily discriminated from the difference in the way of visual recognition when observing a cross section of the battery case 10 including the welded portion 21 as schematically shown in FIGS. 4A and 4B with a scanning electron microscope (SEM) (for example, JSM-7000F manufactured by JEOL Ltd.). Although FIGS. 4A and 4B illustrate the case where the heat-affected zone 203 exists, there may be a case where the heat-affected zone 203 does not exist when observing a cross section of the battery case 10 including the welded portion 21 with SEM. In the present embodiment, it is assumed that the welded portion 21 does not include the heat-affected zone 203 as described above.

[0038] Here, the weld metal 201 is a metal obtained by melting the steel material, which is the material of the case main body portion 11 and the lid portion 13, during welding and then solidifying again. Therefore, the weld metal 201 is composed of various alloys of chemical components derived from the steel material. Here, the chemical components contained in the steel material will be described in detail again below. Further, the weld metal 201 may contain impurities in addition to the above components.

[0039] Furthermore, the chemical components constituting the weld metal 201 can be identified by observing a cross-section obtained by cutting the portion corresponding to the weld metal 201 using an electron probe microanalyzer (EPMA) mounted on a scanning electron microscope (for example, JXA-8230 manufactured by JEOL Corporation).

[0040] More specifically, a sample for cross-sectional observation, including the welded portion 21 of the battery case 10, is cut from the center of the battery case 10 in the depth direction (Y-axis direction in Figure 1). The cutting direction for obtaining the cross-section is the thickness direction of the lid 13 (the longitudinal direction of the battery case 10 (Z-axis direction in Figure 1)). The obtained cross-sectional observation sample is embedded in resin so that the cross-section in the thickness direction is visible, and then the cross-section is polished. Afterward, the obtained cross-section (polished surface) can be observed using SEM-EPMA. The acceleration voltage is set to 15 kV, and the irradiation current to 0.05 μA. Furthermore, during observation, a 1 mm × 1 mm area of ​​the cross-section is quantitatively analyzed, and the average value of 10 points separated by at least 5 μm from each other is taken as the chemical composition of the weld metal. If the chemical composition of the weld metal contains multiple elements, in this embodiment, the weld metal is assumed to be composed of the alloy contained in the materials of the case body and lid.

[0041] [Regarding the oxide film layer 205] In addition, in the battery case 10 according to this embodiment, the oxide film layer 205 is present on the surface of the weld metal 201 located on the inner surface side of the battery case 10 in the welded portion 21, in the portion that can come into contact with the electrolyte 5. For example, as schematically shown in Figures 4A and 4B, when a part of the case body portion 11 of the welded portion 21 and a part of the lid portion 13 are located corresponding to each other with a gap 23 in between, the oxide film layer 205 is present on the surface of the weld metal 201 facing the gap 23, in the portion that can come into contact with the electrolyte 5.

[0042] Here, the oxide film layer 205 is composed of oxides (e.g., FeO) derived from the steel material that makes up the case body 11 and lid 13. In addition, the oxide film layer 205 may contain impurities other than the oxides mentioned above.

[0043] In the battery case 10 according to this embodiment, the thickness of the oxide film layer 205, as schematically shown in Figures 4A and 4B, is preferably, for example, 0.01 μm or more and 1.00 μm or less. By making the thickness of the oxide film layer 205 0.01 μm or more, it is possible to further improve the electrolyte resistance of the welded portion 21. The thickness of the oxide film layer 205 located on the inner surface side of the battery case 10 in the welded portion 21 is more preferably 0.03 μm or more, and even more preferably 0.05 μm or more.

[0044] On the other hand, by making the thickness of the oxide film layer 205 located on the inner surface side of the battery case 10 in the welded portion 21 1.00 μm or less, it is possible to further improve the electrolyte resistance of the welded portion 21 while preventing the oxide film layer 205 from falling off due to expansion and contraction that occurs during charging and discharging of the lithium-ion battery. The thickness of the oxide film layer 205 located on the inner surface side of the battery case 10 in the welded portion 21 is more preferably 0.50 μm or less, even more preferably 0.40 μm or less, and even more preferably 0.20 μm or less.

[0045] The thickness of the oxide film layer 205 described above can be controlled by controlling the welding speed when welding the case body 11 and the lid 13, whether or not to use shielding gas, the flow rate of the shielding gas, etc.

[0046] The thickness of the oxide film layer 205 described above can be confirmed, for example, as follows. First, a sample for cross-sectional observation, including the welded part 21 of the battery case 10, is cut from the center of the battery case 10 in the depth direction (Y-axis direction in Figure 1). At this time, the cutting direction for obtaining the cross-section is the plate thickness direction of the lid 13 (the longitudinal direction of the battery case 10 (Z-axis direction in Figure 1)). After embedding the obtained cross-sectional observation sample in room-temperature drying epoxy resin, the cross-section is polished. Next, the obtained cross-section can be measured by observing it with a SEM. At this time, the field of view during observation should be 30 μm × 30 μm. More specifically, the cross-section of the cross-sectional observation sample obtained as described above is observed using a SEM-EPMA (for example, JXA-8230 manufactured by JEOL Corporation), and a mapping analysis of the distribution of each element is performed.

[0047] In this observation, a depth profile representing the change in oxygen (O) content is created, with the point of contact with the embedded resin located on the inner surface of the battery case as the origin. The region up to the point where the oxygen content peaks is defined as the area of ​​the oxide film layer 205. Cross-sectional observation samples are taken from 10 arbitrary locations separated by at least 1 cm from each other. For each cross-sectional observation sample, the thickness of the oxide film layer 205, as determined above, is measured using the length-measuring function implemented in the SEM-EPMA. The average value is calculated using the 8 measurement points obtained by excluding the maximum and minimum values ​​from the 10 obtained measurement points. The average value obtained in this way is treated as the thickness of the oxide film layer 205. Alternatively, instead of taking cross-sectional observation samples from 10 arbitrary locations separated by at least 1 cm from each other, 10 measurement points may be obtained as follows: the thickness of the oxide film layer 205 is measured using the length-measuring function for a cross-sectional observation sample taken from one arbitrary location, and then final polishing is performed. The thickness of the oxide film layer 205 is measured in the same manner on the new cross-section obtained by the final polishing. This observation may be carried out in the same manner to obtain 10 measurement points.

[0048] In the SEM-EPMA observation described above, the acceleration voltage is set to 15 kV and the irradiation current to 0.05 μA. Furthermore, during observation, the 30 μm × 30 μm region is observed using a 500 pixel × 500 pixel array, with an irradiation time of 50 ms per point.

[0049] The battery case 10 for the lithium-ion battery 1 according to this embodiment and the configuration of the lithium-ion battery 1 using the battery case 10 have been described in detail above with reference to Figures 1 to 4B.

[0050] <Regarding the steel material used for the case body 11 and lid 13> Next, the steel material used for the case body 11 and lid 13 in the battery case 10 according to this embodiment will be described in detail with reference to Figure 5. Figure 5 is a schematic explanatory diagram showing the material of the battery case for lithium-ion batteries according to this embodiment.

[0051] In this embodiment, the case body 11 and lid 13 are made of steel material 100 as schematically shown in Figure 5.

[0052] ≪Regarding the chemical composition of steel material 100≫ According to one embodiment, the chemical composition of steel material 100 according to this embodiment is as follows, by mass%, containing Cr: 1.0 to 9.9%, Al: 0.5 to 10.0%, Mn: 0.1 to 5.0%, C: 0.002 to 0.100%, Si: 0.01 to 0.50%, P: 0.005 to 0.070%, S: 0.001 to 0.250%, N: 0.001 to 0.020%, B: 0.0001 to 0.0030%, with the remainder being Fe and impurities.

[0053] Furthermore, according to another embodiment, the chemical composition of the steel material 100 according to this embodiment is as follows: in mass%, it contains Cr: 1.0 to 9.9%, Al: 0.5 to 10.0%, Mn: 0.1 to 5.0%, C: 0.002 to 0.100%, Si: 0.01 to 0.50%, P: 0.005 to 0.070%, S: 0.001 to 0.250%, N: 0.001 to 0.020%, B: 0.0001 to 0.0030%, and further contains one or more elements selected from the group consisting of element groups A to D below, with the remainder being Fe and impurities.

[0054] [Element Group A]: One or more elements selected from the group consisting of Sn: 2.0% or less, Ti: 1.0% or less, and Cu: 1.50% or less. [Element Group B]: Nb: 0.200% or less. [Element Group C]: One or two elements selected from the group consisting of Mo: 3.0% or less, and Ni: 9.0% or less. [Element Group D]: One or more elements selected from the group consisting of V: ​​0.10% or less, As: 0.10% or less, Sb: 0.50% or less, Ca: 0.050% or less, and Mg: 0.0500% or less.

[0055] [Cr: 1.0 to 9.9% by mass] Cr is an element necessary to ensure the corrosion resistance (more specifically, external corrosion resistance and electrolyte resistance) required for the battery case 10 in the steel material 100 according to this embodiment, and is included in a predetermined amount or more. If the Cr content in the steel material 100 is less than 1.0% by mass, the above-mentioned corrosion resistance cannot be guaranteed. For this reason, the Cr content in the steel material 100 is 1.0% by mass or more. The Cr content is preferably 2.0% by mass or more, and more preferably 3.0% by mass or more.

[0056] On the other hand, if the Cr content of the steel material 100 exceeds 9.9% by mass, not only will the manufacturing cost of the battery case 10 increase, but the processability of the steel material 100 when processing it into the battery case 10 (case body 11 and lid 13) will decrease. A decrease in processability is undesirable because it reduces the yield of the battery case 10. Furthermore, if the Cr content of the steel material 100 exceeds 9.9% by mass, Cr carbides will form in the steel material 100, and the external corrosion resistance when the steel material 100 is processed into the battery case 10 will also decrease. For this reason, the Cr content of the steel material 100 should be 9.9% by mass or less. Preferably, the Cr content is 7.0% by mass or less, and more preferably 6.0% by mass or less.

[0057] [Al: 0.5 to 10.0 mass%] Al is an element necessary to ensure the corrosion resistance (more specifically, external corrosion resistance and electrolyte resistance) required for the battery case 10 in the steel material 100 according to this embodiment, and is included in a predetermined or higher content. If the Al content in the steel material 100 is less than 0.5 mass%, the above-mentioned corrosion resistance cannot be guaranteed. For this reason, the Al content in the steel material 100 is 0.5 mass% or more. The Al content is preferably 0.8 mass% or more, and more preferably 1.0 mass% or more.

[0058] On the other hand, if the Al content of the steel material 100 exceeds 10.0% by mass, not only will the manufacturing cost of the battery case 10 increase, but the processability of the steel material 100 when processing it into the battery case 10 (case body 11 and lid 13) will decrease. A decrease in processability is undesirable because it reduces the yield of the battery case 10. Therefore, the Al content of the steel material 100 should be 10.0% by mass or less. Preferably, the Al content is 5.0% by mass or less, and more preferably 3.0% by mass or less.

[0059] [Mn: 0.1 to 5.0 mass%] Mn is an element that lowers the Ac3 transformation point of steel material 100. By including Mn in steel material 100 at a predetermined or higher concentration, it becomes possible to control the Ac3 transformation point of steel material 100 within a desired range, thereby enabling appropriate control of the mechanical strength of steel material 100. In order to achieve this Ac3 transformation point control effect, the Mn content in steel material 100 is set to 0.1 mass% or more.

[0060] Furthermore, Mn is a useful element as a deoxidizing agent for steel. The effect of Mn as a deoxidizing agent for steel can be achieved by setting the Mn content to 1.0 mass%. Therefore, in order to achieve both the effect of controlling the Ac3 transformation point and the effect as a deoxidizing agent, it is preferable that the Mn content in the steel material 100 be 1.0 mass% or more. More preferably, the Mn content in the steel material 100 is 2.0 mass% or more.

[0061] On the other hand, if the Mn content of the steel material 100 exceeds 5.0 mass%, the Ac3 transformation point control effect saturates, while the manufacturing cost of the battery case 10 increases, which is undesirable. Therefore, the Mn content of the steel material 100 should be 5.0 mass% or less. Furthermore, by setting the Mn content of the steel material 100 to 3.0 mass% or less, it is possible to maintain the workability of the steel material 100 while simultaneously exhibiting both the Ac3 transformation point control effect and the deoxidizing effect, which is preferable. More preferably, the Mn content of the steel material 100 is 2.7 mass% or less.

[0062] [C: 0.002 to 0.100 mass%] Since carbon (C) is an element that dissolves in steel and improves its strength, it is included in a predetermined amount or more. If the C content in steel material 100 is less than 0.002 mass%, the above-mentioned strength improvement effect cannot be achieved. For this reason, the C content in steel material 100 is 0.002 mass% or more. Furthermore, in order to achieve both the strength of the steel and the assurance of workability, the C content in steel material 100 is preferably more than 0.020 mass%, and more preferably 0.010 mass% or more. The C content in steel material 100 is even more preferably 0.020 mass% or more.

[0063] On the other hand, if the carbon content of the steel material 100 exceeds 0.100% by mass, carbon forms carbides in the steel, reducing the corrosion resistance of the steel material 100, which is undesirable. Also, when manufacturing battery cases for lithium-ion batteries, the case body is sometimes made by welding, and from the viewpoint of welding, a lower carbon content is preferable. For this reason, the carbon content of the steel material 100 is set to 0.100% by mass or less. Preferably, the carbon content of the steel material 100 is 0.050% by mass or less, and more preferably 0.030% by mass or less.

[0064] [Si: 0.01 to 0.50 mass%] Since Si is an element that improves the strength of steel, it is included in a predetermined amount or more. If the Si content in the steel material 100 is less than 0.01 mass%, the strength improvement effect described above cannot be achieved. For this reason, the Si content in the steel material 100 is 0.01 mass% or more. Furthermore, in order to achieve both the strength of the steel and the assurance of workability, the Si content in the steel material 100 is preferably 0.10 mass% or more, and more preferably 0.25 mass% or more.

[0065] On the other hand, if the Si content in the steel material 100 is excessive, Si will concentrate on the steel surface during heating in the manufacturing process, reducing the electrolyte resistance of the manufactured steel material 100. This surface concentration of Si becomes noticeable when the Si content of the steel material 100 exceeds 0.50% by mass. Therefore, the Si content in the steel material 100 should be 0.50% by mass or less. To more appropriately ensure the electrolyte resistance of the steel material 100, the Si content in the steel material 100 is preferably less than 0.50% by mass, more preferably 0.40% by mass or less, and even more preferably 0.30% by mass or less.

[0066] [P: 0.005 to 0.070 mass%] P is an element that is inevitably contained in steel. Although it is possible to reduce the P content in steel by dephosphorization treatment in the steelmaking process, reducing the P content to less than 0.005 mass% is undesirable because it excessively increases the manufacturing cost of the steel. From this viewpoint, the P content in steel material 100 is set to 0.005 mass% or more.

[0067] Furthermore, phosphorus (P) is an element that improves the strength of steel. The strength-improving effect of P becomes significant when the P content is 0.010% by mass or more, so it is preferable that the P content in steel material 100 is 0.010% by mass or more. More preferably, the P content in steel material 100 is 0.020% by mass or more.

[0068] On the other hand, if the P content of the steel material 100 exceeds 0.070 mass%, the strength of the steel material 100 increases too much, resulting in a decrease in workability, which is undesirable. Therefore, the P content in the steel material 100 should be 0.070 mass% or less. Preferably, the P content in the steel material 100 is 0.060 mass% or less, and more preferably 0.050 mass% or less.

[0069] [S: 0.001 to 0.250 mass%] S is an element that is inevitably contained in steel. Although it is possible to reduce the S content in steel by desulfurization treatment in the steelmaking process, it is undesirable to reduce the S content to less than 0.001 mass% because it would excessively increase the manufacturing cost of the steel. Therefore, the S content in steel material 100 is set to 0.001 mass% or more. The S content of steel material 100 is preferably 0.002 mass% or more, and more preferably 0.003 mass% or more.

[0070] On the other hand, if the sulfur content of the steel material 100 exceeds 0.250% by mass, the steel material becomes brittle, which is undesirable. Therefore, the sulfur content of the steel material 100 should be 0.250% by mass or less. Preferably, the sulfur content of the steel material 100 is 0.100% by mass or less, and more preferably 0.010% by mass or less.

[0071] [N: 0.001 to 0.020 mass%] N is an element that is inevitably contained in steel. Reducing the N content to less than 0.001 mass% is undesirable because it excessively increases the manufacturing cost of the steel. Therefore, the N content in steel material 100 is set to 0.001 mass% or more. The N content of steel material 100 is preferably 0.002 mass% or more, and more preferably 0.003 mass% or more.

[0072] On the other hand, if the N content of the steel material 100 exceeds 0.020 mass%, nitrides and carbonitrides are formed in the steel, which degrades the quality of the steel, and is therefore undesirable. For this reason, the N content of the steel material 100 should be 0.020 mass% or less. Preferably, the N content of the steel material 100 is 0.010 mass% or less, and more preferably 0.005 mass% or less.

[0073] [B: 0.0001 to 0.0030 mass%] B is an element that refines the recrystallized grains during annealing in the manufacture of steel. By refining the recrystallized grains, the rollability of the steel is improved when cold-rolled, thus improving the productivity of the steel 100. This recrystallized grain refinement effect becomes significant when the B content of the steel 100 is 0.0001 mass% or more. Therefore, the B content of the steel is set to 0.0001 mass% or more. The B content of the steel 100 is preferably 0.0005 mass% or more, and more preferably 0.0010 mass% or more.

[0074] On the other hand, if the B content of the steel material 100 exceeds 0.0030 mass%, the optimal rolling rate due to refinement decreases, and the productivity of the steel material 100 decreases. For this reason, the B content of the steel material 100 should be 0.0030 mass% or less. Preferably, the B content of the steel material 100 is 0.0025 mass% or less, and more preferably 0.0020 mass% or less.

[0075] In the steel material 100 according to this embodiment, the remainder of the above Cr, Al, Mn, C, Si, P, S, N, and B is Fe and impurities. Here, "impurities" means components that are mixed into the steel material during the industrial production of steel material due to raw materials such as ore and scrap, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the various properties of the steel material according to this embodiment.

[0076] Because the steel material 100 according to this embodiment has the above-described chemical composition, the steel material 100 according to this embodiment can exhibit even better electrolyte resistance.

[0077] Next, in a steel material 100 according to another embodiment of this invention, element groups A to D, which may be present in the chemical composition of the steel material 100, will be described in detail.

[0078] In addition, in the steel material 100 according to another embodiment of this embodiment, if at least one of the elements belonging to element groups A to D below is included, it is preferable that at least one of the elements belonging to element groups A to D below is included within the following content range, and the total content is 10.00% by mass or less.

[0079] By keeping the total content of elements belonging to element groups A to D to 10.00% by mass or less, it becomes possible to enjoy the effects exhibited by the addition of each element, as detailed below, without impairing each other. The total content of elements belonging to element groups A to D is preferably 8.00% by mass or less, and more preferably 5.00% by mass or less.

[0080] ◇Element Group A In another embodiment of the steel material 100 according to this embodiment, element group A that the steel material 100 may contain will be described below. At least one of the elements of element group A shown below may be contained in the steel material 100 in place of a portion of the remaining Fe. [Element Group A]: One or more elements selected from the group consisting of Sn: 2.0% or less, Ti: 1.0% or less, and Cu: 1.50% or less.

[0081] [Sn: 0 to 2.0 mass%] In the steel material 100 according to this embodiment, it is possible to use it as a steel material even without containing Sn, so the lower limit of its content is 0 mass%. On the other hand, Sn is an element that can improve the corrosion resistance (especially the external corrosion resistance) of the steel material 100 under various corrosive environments. For this reason, the Sn content in the steel material 100 according to this embodiment can be greater than 0 mass%. This effect of improving corrosion resistance is manifested when the Sn content in the steel material 100 is 0.1 mass% or more. For this reason, when Sn is included in the steel material 100, it is preferable that the Sn content be 0.1 mass% or more. More preferably, the Sn content in the steel material 100 is 0.2 mass% or more.

[0082] On the other hand, if the Sn content in the steel material 100 exceeds 2.0% by mass, the above-mentioned effect of improving corrosion resistance saturates, and the manufacturing cost of the steel material increases. Therefore, when Sn is included in the steel material 100, it is preferable that the Sn content be 2.0% by mass or less. More preferably, the Sn content in the steel material 100 is 1.0% by mass or less.

[0083] [Ti: 0 to 1.0 mass%] In the steel material 100 according to this embodiment, it is possible that it does not contain Ti, so the lower limit of its content is 0 mass%. On the other hand, Ti is an element that suppresses the formation of Cr carbides by forming carbides with C in the steel material 100, thereby preventing a decrease in the corrosion resistance (particularly external corrosion resistance) of the steel material 100. For this reason, in the steel material 100 according to this embodiment, the Ti content may be greater than 0 mass%. This effect is manifested when the Ti content in the steel material 100 is 0.1 mass% or more. For this reason, when Ti is included in the steel material 100, it is preferable that the Ti content be 0.1 mass% or more. More preferably, the Ti content in the steel material 100 is 0.3 mass% or more.

[0084] On the other hand, if the Ti content in the steel material 100 exceeds 1.0 mass%, the effect of preventing the decrease in corrosion resistance described above becomes saturated, and the manufacturing cost of the steel material increases. Therefore, when Ti is included in the steel material 100, it is preferable that the Ti content be 1.0 mass% or less. More preferably, the Ti content in the steel material 100 is 0.7 mass% or less.

[0085] [Cu: 0 to 1.50 mass%] In the steel material 100 according to this embodiment, it is possible that it does not contain Cu, so the lower limit of its content is 0 mass%. On the other hand, Cu is an element that improves the corrosion resistance (especially external corrosion resistance) of steel materials containing Cr. Therefore, in the steel material 100 according to this embodiment, the Cu content may be greater than 0 mass%. This effect of improving corrosion resistance is exhibited when the Cu content in the steel material 100 is 0.01 mass% or more. Therefore, when Cu is included in the steel material 100, it is preferable that the Cu content be 0.01 mass% or more. More preferably, the Cu content in the steel material 100 is 0.02 mass% or more.

[0086] On the other hand, if the Cu content in the steel material 100 exceeds 1.50% by mass, the above-mentioned effect of improving corrosion resistance saturates, and the manufacturing cost of the steel material increases. Therefore, when Cu is included in the steel material 100, it is preferable that the Cu content be 1.50% by mass or less. More preferably, the Cu content in the steel material 100 is 1.00% by mass or less.

[0087] ◇Element Group B In another embodiment of the steel material 100 according to this embodiment, element group B that the steel material 100 may contain will be described. The elements of element group B shown below are elements that may be contained in the steel material 100 in place of a portion of the remaining Fe. [Element Group B]: Nb: 0.200% or less

[0088] [Nb: 0 to 0.200 mass%] In the steel material 100 according to this embodiment, it is possible that it does not contain Nb, so the lower limit of its content is 0 mass%. On the other hand, Nb is an element that forms fine carbides in steel and improves the toughness of steel through its grain-refining effect. Therefore, in the steel material 100 according to this embodiment, the Nb content may be greater than 0 mass%. This toughness improvement effect is manifested when the Nb content in the steel material 100 is 0.050 mass% or more. Therefore, when Nb is included in the steel material 100, it is preferable that the Nb content be 0.050 mass% or more. More preferably, the Nb content in the steel material 100 is 0.070 mass% or more.

[0089] On the other hand, if the Nb content in the steel material 100 exceeds 0.200 mass%, the carbides produced become coarser, and the toughness of the steel decreases. Therefore, when Nb is included in the steel material 100, it is preferable that the Nb content be 0.200 mass% or less. More preferably, the Nb content in the steel material 100 is 0.080 mass% or less.

[0090] ◇Element Group C In another embodiment of the steel material 100 according to this embodiment, element group C that the steel material 100 may contain will be described. At least one of the elements of element group C shown below may be contained in the steel material 100 in place of a portion of the remaining Fe. [Element Group C]: One or two elements selected from the group consisting of Mo: 3.0% or less and Ni: 9.0% or less

[0091] [Mo: 0 to 3.0 mass%] In the steel material 100 according to this embodiment, it is possible that it does not contain Mo, so the lower limit of its content is 0 mass%. On the other hand, Mo is an element that improves the electrolyte resistance of the steel material 100. For this reason, in the steel material 100 according to this embodiment, the Mo content may be greater than 0 mass%. This effect of improving electrolyte resistance is manifested when the Mo content in the steel material 100 is 0.2 mass% or more. For this reason, when Mo is included in the steel material 100, it is preferable that the Mo content be 0.2 mass% or more. More preferably, the Mo content in the steel material 100 is 0.5 mass% or more.

[0092] On the other hand, if the Mo content in the steel material 100 exceeds 3.0% by mass, the above-mentioned effect of improving electrolyte resistance saturates, and the manufacturing cost of the steel material increases. Therefore, when Mo is included in the steel material 100, it is preferable that the Mo content be 3.0% by mass or less. More preferably, the Mo content in the steel material 100 is 2.0% by mass or less.

[0093] [Ni: 0 to 9.0 mass%] In the steel material 100 according to this embodiment, it is possible that it does not contain Ni, so the lower limit of its content is 0 mass%. On the other hand, Ni is an element that improves the electrolyte resistance of the steel material 100. For this reason, in the steel material 100 according to this embodiment, the Ni content may be greater than 0 mass%. This effect of improving electrolyte resistance is manifested when the Ni content in the steel material 100 is 1.0 mass% or more. For this reason, when Ni is included in the steel material 100, it is preferable that the Ni content be 1.0 mass% or more. More preferably, the Ni content in the steel material 100 is 3.0 mass% or more.

[0094] On the other hand, if the Ni content in the steel material 100 exceeds 9.0% by mass, the above-mentioned effect of improving electrolyte resistance saturates, and the manufacturing cost of the steel material increases. Therefore, when Ni is included in the steel material 100, it is preferable that the Ni content be 9.0% by mass or less. More preferably, the Ni content in the steel material 100 is 6.0% by mass or less.

[0095] ◇Element Group D In another embodiment of the steel material 100 according to this embodiment, element group D that the steel material 100 may contain will be described. At least one of the elements of element group D shown below may be contained in the steel material 100 in place of a portion of the remaining Fe. [Element Group D]: One or more elements selected from the group consisting of V: ​​0.10% or less, As: 0.10% or less, Sb: 0.50% or less, Ca: 0.050% or less, and Mg: 0.0500% or less.

[0096] [V: 0 to 0.10 mass%] In the steel material 100 according to this embodiment, it is possible that V may not be contained, so the lower limit of its content is 0 mass%. On the other hand, V is an element that improves the corrosion resistance of the processed part (processed part) in a member obtained by processing the steel material 100. For this reason, in the steel material 100 according to this embodiment, the V content may be greater than 0 mass%. This effect of improving the corrosion resistance of the processed part is manifested when the V content in the steel material 100 is 0.01 mass% or more. For this reason, when V is included in the steel material 100, it is preferable that the V content be 0.01 mass% or more. More preferably, the V content in the steel material 100 is 0.04 mass% or more.

[0097] On the other hand, if the V content in the steel material 100 exceeds 0.10% by mass, the above-mentioned effect of improving the corrosion resistance of the processed part saturates, and the manufacturing cost of the steel material increases. Therefore, when V is included in the steel material 100, it is preferable that the V content be 0.10% by mass or less. More preferably, the V content in the steel material 100 is 0.08% by mass or less.

[0098] [As: 0 to 0.10 mass%] In the steel material 100 according to this embodiment, it is possible that it does not contain As, so the lower limit of its content is 0 mass%. On the other hand, As is an element that improves the corrosion resistance of the processed part in a component obtained by processing the steel material 100. For this reason, in the steel material 100 according to this embodiment, the As content may be greater than 0 mass%. This effect of improving the corrosion resistance of the processed part is manifested when the As content in the steel material 100 is 0.01 mass% or more. For this reason, when As is included in the steel material 100, it is preferable that the As content be 0.01 mass% or more. More preferably, the As content in the steel material 100 is 0.02 mass% or more.

[0099] On the other hand, if the As content in the steel material 100 exceeds 0.10% by mass, the above-mentioned effect of improving the corrosion resistance of the processed part saturates, and the manufacturing cost of the steel material increases. Therefore, when As is included in the steel material 100, it is preferable that the As content be 0.10% by mass or less. More preferably, the As content in the steel material 100 is 0.06% by mass or less.

[0100] [Sb: 0 to 0.50 mass%] In the steel material 100 according to this embodiment, it is possible that it does not contain Sb, so the lower limit of its content is 0 mass%. On the other hand, Sb is an element that improves the corrosion resistance of the processed part in a member obtained by processing the steel material 100. For this reason, in the steel material 100 according to this embodiment, the Sb content may be greater than 0 mass%. This effect of improving the corrosion resistance of the processed part is manifested when the Sb content in the steel material 100 is 0.01 mass% or more. For this reason, when Sb is included in the steel material 100, it is preferable that the Sb content be 0.01 mass% or more. More preferably, the Sb content in the steel material 100 is 0.02 mass% or more.

[0101] On the other hand, if the Sb content in the steel material 100 exceeds 0.50% by mass, the above-mentioned effect of improving the corrosion resistance of the processed part will saturate, and the manufacturing cost of the steel material will increase. For this reason, when Sb is included in the steel material 100, it is preferable that the Sb content be 0.50% by mass or less. More preferably, the Sb content in the steel material 100 is 0.30% by mass or less.

[0102] [Ca: 0 to 0.050 mass%] In the steel material 100 according to this embodiment, it is possible that it does not contain Ca, so the lower limit of its content is 0 mass%. On the other hand, Ca is an element that improves the corrosion resistance of the processed part in a component obtained by processing the steel material 100. For this reason, in the steel material 100 according to this embodiment, the Ca content may be greater than 0 mass%. This effect of improving the corrosion resistance of the processed part is manifested when the Ca content in the steel material 100 is 0.001 mass% or more. For this reason, when Ca is included in the steel material 100, it is preferable that the Ca content be 0.001 mass% or more. More preferably, the Ca content in the steel material 100 is 0.005 mass% or more.

[0103] On the other hand, if the Ca content in the steel material 100 exceeds 0.050 mass%, the above-mentioned effect of improving the corrosion resistance of the processed part will saturate, and the manufacturing cost of the steel material will increase. For this reason, when Ca is included in the steel material 100, it is preferable that the Ca content be 0.050 mass% or less. More preferably, the Ca content in the steel material 100 is 0.010 mass% or less.

[0104] [Mg: 0 to 0.0500 mass%] In the steel material 100 according to this embodiment, it is possible that it does not contain Mg, so the lower limit of its content is 0 mass%. On the other hand, Mg is an element that improves the corrosion resistance of the processed part in a component obtained by processing the steel material 100. For this reason, the Mg content in the steel material 100 according to this embodiment may be greater than 0 mass%. This effect of improving the corrosion resistance of the processed part is manifested when the Mg content in the steel material 100 is 0.0001 mass% or more. For this reason, when Mg is included in the steel material 100, it is preferable that the Mg content be 0.0001 mass% or more. More preferably, the Mg content in the steel material 100 is 0.0010 mass% or more.

[0105] On the other hand, if the Mg content in the steel material 100 exceeds 0.0500 mass%, the above-mentioned effect of improving the corrosion resistance of the processed part will saturate, and the manufacturing cost of the steel material will increase. For this reason, when Mg is included in the steel material 100, it is preferable that the Mg content be 0.0500 mass% or less. More preferably, the Mg content in the steel material 100 is 0.0100 mass% or less.

[0106] [Method for Measuring Chemical Composition] The chemical composition of the steel material 100 can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry). For the steel material 100 of interest, a position corresponding to 1 / 4 of the plate thickness d is identified along the plate thickness direction from the surface, and a measurement sample is taken from this position. When taking a measurement sample from the member, the sample should be taken from a location sufficiently far from welds and bends (for example, at least 10 mm away from the end of a weld, and at least 10 mm away from the end of a bend). By performing measurements on the acquired measurement sample in accordance with JIS G 1258-1:2014, etc., the content of elements other than C, N, and O in the chemical composition of the steel material 100 can be determined.

[0107] The carbon content of the steel material 100 of interest can be determined by measuring a sample obtained in the same manner using the so-called combustion-infrared absorption method. The nitrogen content of the steel material 100 of interest can be determined by measuring a sample obtained in the same manner using the so-called inert gas fusion-thermal conductivity method. The oxygen content of the steel material 100 of interest can be determined by measuring a sample obtained in the same manner using the so-called inert gas fusion-nondispersive infrared absorption method.

[0108] As described above, by keeping the chemical composition of the steel material 100 used for the battery case 10 according to this embodiment within a specific range, it is possible to achieve not only superior external corrosion resistance but also superior electrolyte resistance. Therefore, by using the steel material 100 according to this embodiment, it is possible to use the steel material 100 as the material for the battery case 10 without adding any new plating layers to the surface of the steel material 100. As a result, in this embodiment, the manufacturing cost of producing the battery case 10 can be further reduced.

[0109] The chemical composition of the steel material 100 according to this embodiment has been described in detail above.

[0110] ≪Regarding the presence of fluoride on the inner surface of the battery case 10≫ As described above, the steel material 100 that forms the case body 11 and lid 13 of the battery case 10 according to this embodiment contains Cr and Al in specific amounts as its chemical components. During the process from manufacturing the case body 11 and lid 13 of the battery case 10 using the above-mentioned steel material 100 as the material to assembling them to form a lithium-ion battery, a native oxide film containing oxides of Cr and Al comes into existence on the surface of the case body 11 and lid 13 due to the reaction of oxygen in the air with Cr and Al in the steel material 100. This native oxide film is often a thin film with a thickness of less than 0.01 μm. In addition, in the welded part 21 described earlier, an oxide film layer 205 is formed during welding.

[0111] On the other hand, lithium salts containing fluorine are generally used as the electrolyte in lithium-ion batteries. The highly reactive fluoride ions contained in the electrolyte react with the native oxide film or oxide film layer 205, and the chromium oxides and aluminum oxides contained in the native oxide film or oxide film layer 205 react with the fluoride ions to form chromium fluoride (CrF). ３ ) and Al fluoride (AlF ３ ) is formed. Thus, on the inner surface of the battery case 10 according to this embodiment, at least one of Cr fluoride or Al fluoride is present in the portion that comes into contact with the electrolyte.

[0112] The above-mentioned Cr fluoride and Al fluoride are also stable compounds with respect to the lithium salts containing fluorine included in the electrolytic solution. Since these fluorides are present on the surface of the portion in contact with the electrolytic solution on the inner surface side of the battery case 10, the battery case 10 according to the present embodiment exhibits more excellent electrolytic solution resistance.

[0113] Here, whether the above-mentioned Cr fluoride and Al fluoride are present in the portion in contact with the electrolytic solution on the inner surface side of the battery case 10 can be determined by collecting a measurement sample from the battery case 10 of the portion in contact with the electrolytic solution and performing the following measurements.

[0114] First, after completely discharging the lithium ion battery having the battery case 10 of interest, the case main body portion 11 or the lid portion 13 is cut to separate the battery unit and the battery case 10. From the obtained battery case 10, a 50 mm × 50 mm sample is collected, and the surface corresponding to the inner surface side of the battery case 10 may be analyzed by X-ray photoelectron spectroscopy (XPS). In the obtained F1s spectrum, when peaks of CrF ３ and AlF ３ are present, it can be determined that Cr fluoride and Al fluoride are present. In the F1s spectrum, the peak near the binding energy of 685 eV can be regarded as being derived from CrF ３ , and by observing such a peak, it can be determined that CrF ３ is present. Also, the peak near the binding energy of 686 to 689 eV can be regarded as being derived from AlF ３ , and by observing a peak in such a range, it can be determined that AlF ３ is present.

[0115] Regarding the thickness of the steel material 100 used for the case body 11 and lid 13: In the battery case 10 according to this embodiment, since the steel material described above is used as the material for the case body 11 and lid 13, it has superior mechanical strength compared to the aluminum material used conventionally. Therefore, even if the steel material 100 is thin, it is possible to satisfy the strength required for the battery case 10, which enables miniaturization (thinning) of lithium-ion batteries and reduces manufacturing costs.

[0116] More specifically, when the above-mentioned steel material 100 is used as the material for the case body portion 11, the thickness of the case body portion 11 is preferably in the range of 0.10 to 1.00 mm. More preferably, the thickness of the case body portion 11 is 0.25 mm or more. Furthermore, more preferably, the thickness of the case body portion 11 is 0.60 mm or less.

[0117] Furthermore, when the above-mentioned steel material 100 is used as the material for the lid portion 13, the thickness of the lid portion 13 is preferably in the range of 0.10 to 1.00 mm. More preferably, the thickness of the lid portion 13 is 0.30 mm or more. More preferably, the thickness of the lid portion 13 is 0.80 mm or less.

[0118] The case body 11 and the lid 13 may have the same thickness, or they may have different thicknesses.

[0119] Here, the thickness of the case body 11 and the lid 13 can be measured by taking a measurement sample measuring 10 mm x 20 mm in size in a plan view from approximately the center of the case body 11 and the lid 13, while avoiding the location of the welded part, then embedding and polishing it in resin and observing the cross-section.

[0120] The battery case and lithium-ion battery according to this embodiment have been described in detail above with reference to Figures 1 to 5.

[0121] (Regarding the battery case for lithium-ion batteries and the method for manufacturing lithium-ion batteries) Next, an example of a battery case for lithium-ion batteries according to this embodiment and a method for manufacturing lithium-ion batteries using such a battery case will be described. In the following, the explanation will focus on the battery case 10 shown in Figures 1A and 2A.

[0122] Prior to the following explanation, it is assumed that the case body portion 11 and the lid portion 13 of the battery case 10 have been manufactured using the steel material 100 to achieve the desired shape.

[0123] In this case, the case body portion 11 may be formed so that there are no joints by using a processing method such as drawing. Alternatively, the case body portion 11 may be formed by preparing parts for forming the case body portion 11 (for example, multiple parts for forming the sides of the case body portion 11, or parts for forming the bottom surface of the case body portion 11) using a processing method such as bending, and then laser welding these parts together.

[0124] In the manufacturing method of a lithium-ion battery 1 using the lithium-ion battery case 10 described above, first, the battery unit 3 is placed inside the battery case 10, and then the electrolyte 5 is injected into the battery case 10. After that, the case body 11 and the lid 13 of the battery case 10 are welded together using various welding methods, including laser welding.

[0125] Here, the process of storing the battery unit 3 and electrolyte 5 as described above is carried out using, for example, Ar or N ２ It is preferable to carry out the process under an inert gas atmosphere, or under an atmosphere where moisture content is kept to a minimum (for example, an atmosphere with a dew point of -75°C or lower and a moisture content of 1 ppm or less). Regarding the detailed method of housing the battery unit 3 in the battery case 10 and the detailed method of injecting the electrolyte 5 into the battery case 10, various known methods may be used as appropriate.

[0126] Furthermore, the welding work between the case body 11 and the lid 13 of the battery case 10 is carried out under an inert gas atmosphere, or under an atmosphere that minimizes moisture (for example, an atmosphere with a dew point of -75°C or lower and a moisture content of 1 ppm or less), after the internal space of the battery case 10 has been replaced with an inert gas. Here, the inert gas is Ar gas, N ２ Examples include gas, carbon dioxide, helium, or a mixture of at least two of these gases.

[0127] More specifically, for example, the inert gas is blown into the internal space of the battery case 10 immediately before welding. Then, during welding, the inert gas is blown from above the battery case 10 while the welding is being performed. In this case, the amount of inert gas blown in should be 0.5 L / min or more, and the amount blown from above should be 20.0 L / min or more. The amount of inert gas blown in is preferably 0.6 L / min or more, and more preferably 0.7 L / min or more. The amount blown from above is preferably 25.0 L / min or more, and more preferably 30.0 L / min or more. This fills the internal space of the battery case 10 with inert gas, making it possible to create a situation where the surface of the weld metal 201 produced by welding is less likely to oxidize. The upper limit of the amount of gas blown in is not specifically defined, but in practice it is 1.0 L / min. The upper limit of the amount blown from above is not specifically defined, but in practice it is about 50.0 L / min.

[0128] Furthermore, the method for filling the internal space of the battery case 10 with an inert gas is not limited to blowing in an inert gas as described above.

[0129] By performing the work in an inert gas atmosphere as described above, even if voids 23 exist, the presence of oxygen can be sufficiently removed from the voids 23, and the thickness of the formed oxide film layer 205 can be controlled to a desired state.

[0130] Furthermore, there are no specific limitations on the welding conditions; the conditions should be adjusted as appropriate according to the material being used.

[0131] The battery case for a lithium-ion battery according to this embodiment and a method for manufacturing a lithium-ion battery using such a battery case have been described above.

[0132] (Test Example) In the test example shown below, a case body, lid, and liquid injection port cover, as shown in Figure 1, were prepared, along with the battery unit shown below. These components were then assembled to create a lithium-ion battery.

[0133] <Steel Materials Used> For the case body, lid, and liquid filling port cover that make up the battery case, steel materials with the steel components shown in Tables 1-1 and 1-2 below (plate thickness: 0.25 to 1.00 mm, manufactured by Nippon Steel Corporation) were prepared. In Tables 1-1 and 1-2 below, blank spaces indicate that the corresponding element was not intentionally added. For comparison, commercially available aluminum plates (plate thickness: 0.60 to 1.00 mm) were also prepared separately.

[0134]

[0135]

[0136] The case body was manufactured by deep drawing, bending and seam welding, or bending and laser welding. When the sides of the case body were manufactured using bending, the bottom was welded to form the case body. The welding conditions for each welding process are as follows.

[0137] <Battery Unit> ◇Positive Electrode Plate Lithium cobalt oxide was used as the positive electrode active material. This positive electrode active material was mixed with acetylene black and polyvinylidene fluoride (PVDF) in a mass ratio of lithium cobalt oxide:acetylene black:PVDF = 10:10:1. After that, it was coated onto an aluminum foil as an aqueous dispersion and dried. The resulting material was rolled to a predetermined thickness and cut to a predetermined size to form the positive electrode plate.

[0138] ◇Negative electrode plate: Amorphous carbon was used as the negative electrode active material. The amorphous carbon was dry-mixed with acetylene black, a conductive material, to form a mixture. Then, N-methyl-2-pyrrolidone (NMP), which is obtained by dissolving polyvinylidene fluoride, was uniformly dispersed in the mixture to create a paste with a mass ratio of carbon:acetylene black:PVDF = 88:5:7. The obtained paste was applied to Cu foil, dried, rolled to a predetermined thickness, and then cut to a predetermined size to form the negative electrode plate.

[0139] ◇Separator A polyethylene microporous membrane was used as the separator.

[0140] <Electrolyte> The electrolyte is a solution (1M-LiPF) prepared by adding 1 mol / L of lithium hexafluorophosphate to a mixture of ethylene carbonate and diethyl carbonate in a 1:1 volume ratio. ６ We used EC:DEC (1:1).

[0141] <Lithium-ion battery manufacturing procedure> After inserting a separator between the positive electrode plate and the negative electrode plate obtained as described above, the material was wound up to form an electrode unit. After flattening the battery unit into a shape that could be inserted into the internal space of the case body, the positive electrode plate was welded to an Al lead and the negative electrode plate to a Ni lead. The Al lead was welded to the positive electrode terminal provided on the lid, and the Ni lead was welded to the negative electrode terminal provided on the lid. Then the case body and the lid were welded together.

[0142] The inside of the battery was dried and moisture removed in an atmosphere with a temperature of 25°C, a dew point of -76°C, and a moisture content of 1 ppm or less. Then, under the same atmosphere, the electrolyte was injected through the injection port provided on the lid.

[0143] Subsequently, under an atmosphere of 25°C, a dew point of -76°C, and a moisture content of 1 ppm or less, the liquid injection port provided on the lid was closed with a liquid injection port cover to obtain a completed lithium-ion battery. Multiple completed lithium-ion batteries obtained in this manner were prepared at each level shown in Tables 2-1 and 2-2 below, and used as samples for the evaluation described below. The welding conditions during welding are as follows. The inert gas used during welding was N ２ It is a gas, and the flow rate of such inert gas is as shown in Tables 2-1 and 2-2 below.

[0144] <Laser Welding Conditions> Fiber laser welding machine used. Shielding gas used: N ２ Output: 1.2–6.0 kW Welding speed: 3–12 m / min Focus shift width: 0 mm For each sample, the conditions were adjusted within the above range to ensure penetration to the back surface. <Seam welding conditions> Overlap: Approximately 3–5 times the plate thickness, and seam welding was performed. Single-phase AC (50 Hz), continuous current Contact surface shape: flat Pressurizing force: 300 kgf (1 kgf is approximately 9.8 N) Welding speed: 6–12 m / min

[0145] Subsequently, the obtained lithium-ion battery was charged at 3.6V in an atmosphere with a temperature of 25°C, a dew point of -76°C, and a moisture content of 1 ppm or less. This process electrolytically removed any remaining moisture from the battery.

[0146] <Confirmation of the welded joint of the lithium-ion battery> For each sample obtained as described above, the welded joint between the case body and the lid was cut using a high-speed precision cutting machine, and a sample for cross-sectional observation was cut out. The obtained cross-sectional observation sample was embedded in resin, wet polished up to #1200 emery paper, and then polished to a mirror finish using diamond polishing (Strüers: DP-suspension), and then etched before being subjected to observation. The etching solution used was aqua regia, and the immersion time in the etching solution was 10 seconds.

[0147] Using the cross-sectional observation samples obtained as described above, the chemical composition of the weld was confirmed using a SEM (JSM-7000F, manufactured by JEOL Corporation) and a SEM-EPMA (JXA-8230, manufactured by JEOL Corporation). The measurement field size was 1 mm × 1 mm, the acceleration voltage was 15 kV, and the irradiation current was 0.05 μA. In addition, during observation, a 1 mm × 1 mm area of ​​the cross-section was quantitatively analyzed, and the average value of 10 points was taken as the chemical composition of the weld metal.

[0148] Furthermore, the thickness of the oxide film layer in the welded area was measured using a cross-sectional observation sample prepared in the same manner as described above, with a SEM (JSM-7000F, JEOL Corporation) and a SEM-EPMA (JXA-8230, JEOL Corporation). The acceleration voltage was set to 15 kV, and the irradiation current to 0.05 μA. For observation, the 30 μm × 30 μm area was observed with a 500 pixel × 500 pixel field of view, and the irradiation time per point was set to 50 ms. The thickness of the oxide film layer in the welded area was evaluated according to the following evaluation criteria. Note that in the case of seam welding, since no oxide film layer is formed on the surface of the welded area, it was evaluated with a score of "E". The obtained results are summarized in Tables 2-1 and 2-2 below. ≪Evaluation Criteria≫ Score "A": Oxide film thickness is 0.05 μm or more and 0.20 μm or less "B": Oxide film thickness is 0.03 μm or more and less than 0.05 μm, or greater than 0.20 μm and 0.4 μm or less "C": Oxide film thickness is 0.01 μm or more and less than 0.03 μm, or greater than 0.4 μm and 0.50 μm or less "D": Oxide film thickness is greater than 0.50 μm and 1.00 μm or less "E": Oxide film thickness is less than 0.01 μm, or greater than 1.00 μm

[0149] <Confirmation of Cr Fluoride and Al Fluoride> For each sample obtained as described above, the sample was cut out from the part that had been in contact with the electrolyte, in accordance with the method explained earlier, and the presence or absence of Cr fluoride and Al fluoride was confirmed. The results obtained are summarized in Tables 2-1 and 2-2 below.

[0150] <Evaluation of the obtained lithium-ion batteries> The obtained lithium-ion batteries were evaluated from the viewpoints of external corrosion resistance, electrolyte resistance, and safety. The results obtained are summarized in Tables 2-1 and 2-2 below.

[0151] [External Corrosion Resistance] For each sample prepared as described above, the welded joint between the case body and the lid was cut using a high-speed precision cutting machine to obtain corrosion resistance test specimens. The size of the corrosion resistance test specimens was 50 mm x 50 mm. A salt spray test (SST) as specified in JIS Z2371:2015 was performed on the obtained corrosion resistance test specimens for 5 hours, and the rate of red rust occurrence was calculated. The obtained red rust occurrence rates were evaluated according to the following evaluation criteria, with scores of "EX", "VG", and "G" being considered passing grades. <<Evaluation Criteria>> Score "EX": Red rust occurrence rate is 0% or more and less than 10% "VG": Red rust occurrence rate is 10% or more and less than 30% "G": Red rust occurrence rate is 30% or more and less than 50% "B": Red rust occurrence rate is 50% or more

[0152] The lithium-ion battery prepared as described above was held at 80°C for 1000 hours. After holding, a portion of the battery case was disassembled in an atmosphere of 25°C with a dew point of -76°C, and the electrolyte was collected using a pipette. The amount of metal leached in the obtained electrolyte (more specifically, the amount of Fe leached) was analyzed using a commercially available ICP mass spectrometer (Agilent 7700x ICP-MS manufactured by Agilent Technologies, Inc.). The obtained amount of metal leached was evaluated according to the following evaluation criteria, with scores of "EX", "VG", and "G" being considered passing grades. <<Evaluation Criteria>> Score "EX": Amount of metal leached is 0 ppm or more and less than 10 ppm "VG": Amount of metal leached is 10 ppm or more and less than 20 ppm "G": Amount of metal leached is 20 ppm or more and less than 50 ppm "B": Amount of metal leached is 50 ppm or more

[0153] [Safety] The safety of the lithium-ion batteries manufactured as described above was verified by a fire spread test, which is detailed below. In the fire spread test, two lithium-ion batteries were prepared for each level shown in Table 2 below, and these two lithium-ion batteries were charged to 4.3V using constant current and constant voltage charging to achieve a State of Charge (SOC) of 110%. The two lithium-ion batteries after charging were designated as the fire spread battery and the ignition battery, respectively.

[0154] A voltmeter was installed on both the ignition battery and the fire-contamination battery to measure the voltage of each battery during the test. A heater was also installed on the side of the ignition battery to allow for heating of the battery. The fire-contamination battery was heated to approximately 50°C using the heater. Then, using two dummy batteries, the batteries were arranged in the sequence (dummy battery / ignition battery / fire-contamination battery / dummy battery) so that adjacent batteries were in contact with each other. After arrangement, the ignition battery was heated to approximately 200°C at a heating rate of 10°C / min to forcibly ignite, and the state was maintained.

[0155] In this fire spread test, the "time required from when the voltage of the ignition battery becomes 0V until the voltage of the fire-spreading battery becomes 0V" was defined as the "fire spread time." The fire spread time was measured for each lithium-ion battery prepared as described above. The obtained fire spread times were evaluated according to the following evaluation criteria, with a score of "G" being considered a pass. <<Evaluation Criteria>> Score "G": Fire spread time of 60 seconds or more "B": Fire spread time of less than 60 seconds

[0156]

[0157]

[0158] As is clear from Tables 2-1 and 2-2 above, the samples corresponding to the embodiments of the present invention passed all evaluation results for external corrosion resistance, electrolyte resistance, and safety, while the samples corresponding to the comparative examples of the present invention failed at least one of the evaluation results for external corrosion resistance, electrolyte resistance, or safety.

[0159] Although preferred embodiments of the present invention have been described in detail above with reference to the attached drawings, the present invention is not limited to these examples. It is clear to any person with ordinary skill in the art to which the present invention belongs that various modifications or alterations can be conceived within the scope of the technical idea described in the claims, and these are also understood to fall within the technical scope of the present invention.

[0160] The embodiments disclosed herein are illustrative and not restrictive in all respects. The embodiments described above may be omitted, replaced, or modified in various ways without departing from the appended claims, the technical scope of the invention as described later, and the spirit thereof. For example, the constituent elements of the embodiments described above can be combined in any way without impairing their effects. Furthermore, such any combination will naturally yield the effects and benefits of each constituent element in the combination, as well as other effects and benefits that will be obvious to those skilled in the art from the description herein.

[0161] Furthermore, the effects described herein are merely descriptive or illustrative, and not limiting. In other words, the technology according to the present invention may produce other effects that will be apparent to those skilled in the art from the description herein, in addition to or instead of the effects described above.

[0162] Furthermore, the following configuration also falls within the technical scope of the present invention. (1) A battery case for a lithium-ion battery comprising: a battery unit having a positive electrode, a negative electrode and a separator; an electrolyte containing a lithium salt; a case body portion that houses the battery unit; and a lid portion that seals the case body portion, wherein the material of the case body portion and the lid portion is steel having a chemical composition in mass%, containing Cr: 1.0 to 9.9%, Al: 0.5 to 10.0%, Mn: 0.1 to 5.0%, C: 0.002 to 0.100%, Si: 0.01 to 0.50%, P: 0.005 to 0.070%, S: 0.001 to 0.250%, N: 0.001 to 0.020%, B: 0.0001 to 0.0030%, with the remainder being Fe and impurities. (1) A battery case for a lithium-ion battery, wherein the battery case has a welded joint where the case body and the lid are welded together. (2) A battery case for a lithium-ion battery, comprising: a battery unit having a positive electrode, a negative electrode and a separator; an electrolyte containing a lithium salt; a case body that houses the battery unit; and a lid that seals the case body, wherein the material of the case body and the lid is, in mass%, Cr: 1.0 to 9.9%, Al: 0.5 to 10.0%, Mn: 0.1 to 5.0%, C: 0.002 to 0.100%, Si: 0.01 to 0.50%, P: 0.005 to 0.070%, S: 0.001 to 0.250%, N: 0.001 to 0.020%, B: 0.0001 to 0.0030%, A battery case for a lithium-ion battery, comprising a steel material having a chemical composition in which it contains, and further contains one or more elements selected from the group consisting of element groups A to D below, with the remainder being Fe and impurities, wherein the battery case has a welded joint where the case body and the lid are welded together.(3) A battery case for a lithium-ion battery according to (2), having a chemical composition containing element group A. (4) A battery case for a lithium-ion battery according to (2), having a chemical composition containing element group B. (5) A battery case for a lithium-ion battery according to (2), having a chemical composition containing element group C. (6) A battery case for a lithium-ion battery according to (2), having a chemical composition containing the element group D. (7) A battery case for a lithium-ion battery according to any one of (1) to (6), wherein the welded portion includes a weld metal composed of an alloy of the chemical components contained in the material of the case body and the lid. (8) A battery case for a lithium-ion battery according to any one of (1) to (7), wherein at least one of Cr fluoride or Al fluoride is present on the inner surface of the battery case in contact with the electrolyte. (9) A battery case for a lithium-ion battery according to any one of (1) to (8), wherein an oxide film layer is present on the inner surface of the battery case in the welded portion in a portion that may come into contact with the electrolyte. (10) A battery case for a lithium-ion battery according to (9), wherein the thickness of the oxide film layer is 0.01 to 1.00 μm. (11) The battery case for a lithium-ion battery according to (10), wherein the thickness of the oxide film layer is 0.01 to 0.50 μm. (12) The battery case for a lithium-ion battery according to any one of (1) to (11), wherein the thickness of the case body is 0.10 to 1.00 mm and the thickness of the lid is 0.10 to 1.00 mm. (13) The battery case for a lithium-ion battery according to any one of (1) to (12), wherein the steel material is a steel material without a plating layer.(14) A battery case for a lithium-ion battery according to any one of (1) to (13), wherein the welded part is a laser-welded part. (15) A lithium-ion battery having a battery case for a lithium-ion battery according to any one of (1) to (14).

[0163] 1 Lithium-ion battery 3 Battery unit 5 Electrolyte 10 Battery case 11 Case body 13 Lid 15 Filling port 17 Filling port cover 21 Welded joint 23 Cavity 100 Steel material 201 Weld metal 203 Heat-affected zone (HAZ) 205 Oxide film layer

Claims

1. A battery case for a lithium-ion battery comprising: a battery unit having a positive electrode, a negative electrode and a separator; an electrolyte containing a lithium salt; a case body for housing the battery unit; and a lid for sealing the case body, wherein the material of the case body and the lid is steel having a chemical composition in mass percent of: Cr: 1.0 to 9.9%, Al: 0.5 to 10.0%, Mn: 0.1 to 5.0%, C: 0.002 to 0.100%, Si: 0.01 to 0.50%, P: 0.005 to 0.070%, S: 0.001 to 0.250%, N: 0.001 to 0.020%, B: 0.0001 to 0.0030%, with the remainder being Fe and impurities. A battery case for a lithium-ion battery, wherein the battery case has a welded joint where the case body and the lid are welded together.

2. A battery case for a lithium-ion battery comprising: a battery unit having a positive electrode, a negative electrode and a separator; an electrolyte containing a lithium salt; a case body portion for housing the battery unit; and a lid portion for sealing the case body portion, wherein the material of the case body portion and the lid portion is, in mass%, Cr: 1.0 to 9.9%, Al: 0.5 to 10.0%, Mn: 0.1 to 5.0%, C: 0.002 to 0.100%, Si: 0.01 to 0.50%, P: 0.005 to 0.070%, S: 0.001 to 0.250%, N: 0.001 to 0.020%, B: 0.0001 to 0.0030%, A battery case for a lithium-ion battery, comprising: a steel material having a chemical composition containing, and further containing one or more elements selected from the group consisting of the following element groups A to D, with the remainder being Fe and impurities, wherein the battery case has a welded joint where the case body and the lid are welded together. [Element group A]: One or more elements selected from the group consisting of Sn: 2.0% or less, Ti: 1.0% or less, and Cu: 1.50% or less [Element group B]: Nb: 0.200% or less [Element group C]: One or two elements selected from the group consisting of Mo: 3.0% or less, and Ni: 9.0% or less [Element group D]: One or more elements selected from the group consisting of V: ​​0.10% or less, As: 0.10% or less, Sb: 0.50% or less, Ca: 0.050% or less, and Mg: 0.0500% or less 3. The battery case for a lithium-ion battery according to claim 1 or 2, wherein the welded portion includes a weld metal composed of an alloy of the chemical components contained in the materials of the case body and the lid.

4. The battery case for a lithium-ion battery according to claim 1 or 2, wherein at least one of Cr fluoride or Al fluoride is present on the inner surface of the battery case in contact with the electrolyte.

5. The battery case for a lithium-ion battery according to claim 1 or 2, wherein in the welded portion, an oxide film layer is present on the inner surface side of the battery case in a portion that may come into contact with the electrolyte.

6. The battery case for a lithium-ion battery according to claim 5, wherein the thickness of the oxide film layer is 0.01 to 1.00 μm.

7. The battery case for a lithium-ion battery according to claim 6, wherein the thickness of the oxide film layer is 0.01 to 0.50 μm.

8. The battery case for a lithium-ion battery according to claim 1 or 2, wherein the thickness of the case body is 0.10 to 1.00 mm, and the thickness of the lid is 0.10 to 1.00 mm.

9. The battery case for a lithium-ion battery according to claim 1 or 2, wherein the steel material is a steel material without a plating layer.

10. The battery case for a lithium-ion battery according to claim 1 or 2, wherein the welded part is a laser-welded part.

11. A battery case for a lithium-ion battery according to claim 2, having a chemical composition containing the element group A.

12. A battery case for a lithium-ion battery according to claim 2, having a chemical composition containing the element group B.

13. A battery case for a lithium-ion battery according to claim 2, having a chemical composition containing the aforementioned element group C.

14. A battery case for a lithium-ion battery according to claim 2, having a chemical composition containing the element group D.

15. A lithium-ion battery having a battery case for a lithium-ion battery according to claim 1 or 2.

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