Secondary battery

Abstract

This secondary battery comprises an electrode body, an exterior can that has an opening and accommodates the electrode body, and a sealing body that seals the opening, the secondary battery being characterized in that: a safety valve that operates when the internal pressure of the battery reaches a prescribed pressure is provided to the bottom of the exterior can; the safety valve has an annular groove (32) provided to the outer surface (16a) of the bottom of the exterior can; the annular groove (32) has a wall surface (36) on the inner peripheral side, a wall surface (38) on the outer peripheral side, and a bottom surface (40) that connects the wall surface (36) on the inner peripheral side and the wall surface (38) on the outer peripheral side; the wall surface (36) on the inner peripheral side is substantially perpendicular to the width direction of the annular groove (36)[GW1.1]; and the wall surface (38) on the outer peripheral side is inclined with respect to the depth direction of the annular groove (36) so that the width of the annular groove (36) becomes narrower toward the bottom surface (40) side.

Description

secondary battery

[0001] This disclosure relates to secondary batteries.

[0002] Rechargeable batteries are generally of the sealed type, in which the opening of the outer casing containing the electrode body is sealed with a sealing body. Conventionally, sealed rechargeable batteries have been equipped with a safety valve that releases gas from inside the battery to the outside when the internal pressure reaches a predetermined pressure, for safety reasons. For example, Patent Document 1 discloses a rechargeable battery in which the safety valve is provided at the bottom of the outer casing. Also, for example, Patent Document 2 discloses a rechargeable battery in which the safety valve is provided on the sealing body. Although not related to the release of gas from inside the battery, for example, Patent Document 3 discloses a device in which a safety valve is provided on a sealed container that houses multiple energy storage devices.

[0003] The safety valve located at the bottom of the outer casing has an annular groove. As the internal pressure of the battery increases, stress is applied to the annular groove. When the internal pressure reaches a predetermined pressure, the groove ruptures, opening the bottom of the area surrounded by the groove and releasing the gas inside the battery. Therefore, to efficiently release the gas inside the battery, it is desirable to increase the diameter of the annular groove and the area surrounded by the groove. However, increasing the diameter of the annular groove reduces the stress on the groove as the internal pressure increases, raising the pressure at which the groove ruptures (the so-called rupture trigger pressure), which poses a safety issue for the battery. One possible solution is to add further grooves, such as a cross shape, to the area surrounded by the annular groove. However, the area surrounded by the annular groove may be processed when the secondary battery is incorporated into a product, in which case grooves cannot be formed within that area.

[0004] Therefore, in order to suppress the rise in the operating pressure for rupture of the annular groove while maintaining a large area within the region surrounded by the annular groove, it is necessary for high stress to be applied to the annular groove when the internal pressure of the battery increases.

[0005] Japanese Patent Publication No. 2012-243715, Japanese Patent Publication No. 2008-251438, Japanese Patent Publication No. 2021-022569

[0006] Therefore, the purpose of this disclosure is to provide a secondary battery equipped with a safety valve that applies high stress to an annular groove when the internal pressure of the battery rises.

[0007] A secondary battery according to one aspect of the present disclosure comprises an electrode body, an outer casing having an opening and housing the electrode body, and a sealing body sealing the opening, wherein a safety valve is provided at the bottom of the outer casing which operates when the internal pressure of the battery reaches a predetermined pressure, and the safety valve has an annular groove provided on the outer surface of the bottom of the outer casing, the annular groove has an inner circumferential wall surface, an outer circumferential wall surface, and a bottom surface connecting the inner circumferential wall surface and the outer circumferential wall surface, the inner circumferential wall surface is substantially perpendicular to the width direction of the annular groove, and the outer circumferential wall surface is inclined with respect to the depth direction of the annular groove such that the width of the annular groove narrows towards the bottom surface.

[0008] According to this disclosure, it is possible to provide a secondary battery equipped with a safety valve that applies high stress to an annular groove when the internal pressure of the battery rises. Furthermore, with a safety valve that applies high stress to an annular groove when the internal pressure of the battery rises, it is possible to suppress the rise in the rupture operating pressure of the annular groove, even when the area of ​​the region surrounded by the annular groove is large, without forming any further grooves within that region.

[0009] This is a cross-sectional view of a secondary battery, which is one example of an embodiment. This is a bottom view of the secondary battery shown in Figure 1. This is an enlarged cross-sectional view of the bottom of the outer casing along the line L-L in Figure 2. This is a bottom view of a secondary battery, which is another example of an embodiment. These are the groove models of the embodiment and the comparative example used in the simulation.

[0010] In the following, an example of an embodiment of the secondary battery according to this disclosure will be described with reference to the drawings. In the following description, the specific shapes, materials, numerical values, directions, etc. are illustrative and can be appropriately changed according to the specifications of the secondary battery.

[0011] Figure 1 is a cross-sectional view of a secondary battery, which is an example of an embodiment. The secondary battery 10 shown in Figure 1 comprises a wound electrode body 14 in which a positive electrode 11 and a negative electrode 12 are wound around a separator 13, an electrolyte, insulating plates 18 and 19 arranged above and below the electrode body 14, a battery case 15, a positive electrode lead 20, and a negative electrode lead 21. Note that other types of electrode bodies may be used instead of the wound electrode body 14, such as a laminated electrode body in which the positive and negative electrodes are alternately stacked with a separator in between. The battery case 15 consists of an outer can 16 having an opening and housing the electrode body 14, etc., and a sealing body 17 that closes the opening of the outer can 16. The battery case 15 is, for example, a cylindrical or rectangular metal case.

[0012] The electrolyte, for example, has lithium ion conductivity. The electrolyte may be a liquid electrolyte (electrolyte solution) or a solid electrolyte.

[0013] A liquid electrolyte (electrolyte solution) comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. Examples of non-aqueous solvents include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixtures thereof. The non-aqueous solvent may also contain halogen-substituted solvents (e.g., fluoroethylene carbonate) in which at least some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. Examples of electrolyte salts include LiPF4. ６ Lithium salts such as these are used.

[0014] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc., can be used. As the inorganic solid electrolyte, materials known for all-solid-state lithium-ion secondary batteries, etc. (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, etc. Although the electrolytes exemplified above are non-aqueous electrolytes, the electrolyte is not limited to non-aqueous electrolytes and may also be an aqueous electrolyte.

[0015] The outer casing 16 is, for example, a metal container with a bottomed cylindrical shape. A gasket 28 is provided between the outer casing 16 and the sealing body 17 to further ensure airtightness inside the battery. The outer casing 16 has, for example, a protruding portion 22 that supports the sealing body 17, where a part of the side surface protrudes inward. The protruding portion 22 is preferably formed in an annular shape along the circumferential direction of the outer casing 16, and its upper surface supports the sealing body 17. Although not shown in Figure 1, a safety valve is provided at the bottom of the outer casing 16. This safety valve will be described later.

[0016] The sealing body 17 has a structure in which a filter 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked in order from the electrode body 14 side. Each component constituting the sealing body 17 has, for example, a disc shape or a ring shape, and each component except the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, with the insulating member 25 interposed between their respective peripheral edges. When the internal pressure of the secondary battery 10 rises due to heat generation caused by an internal short circuit or the like, for example, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 towards the cap 27, thus interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure rises further, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0017] In the secondary battery 10 shown in FIG. 1, the positive electrode lead 20 attached to the positive electrode 11 extends toward the sealing body 17 through the through hole of the insulating plate 18, and the negative electrode lead 21 attached to the negative electrode 12 extends toward the bottom side of the exterior can 16 through the outside of the insulating plate 19. The positive electrode lead 20 is connected by welding or the like to the lower surface of the filter 23 which is the bottom plate of the sealing body 17, and the cap 27 which is the top plate of the sealing body 17 and is electrically connected to the filter 23 serves as the positive electrode terminal. The negative electrode lead 21 is connected by welding or the like to the inner surface of the bottom of the exterior can 16, and the exterior can 16 serves as the negative electrode terminal.

[0018] FIG. 2 is a bottom view of the secondary battery 10 shown in FIG. 1, and is a view of the bottom of the exterior can 16 shown in FIG. 1 as seen from the outside of the secondary battery 10. Further, FIG. 3 is a cross-sectional view of the bottom of the exterior can 16 taken along the line L-L in FIG. 2. A safety valve 30 that operates when the internal pressure of the battery reaches a predetermined pressure is provided at the bottom of the exterior can 16. The safety valve 30 has an annular groove portion 32 provided on the outer surface of the bottom of the exterior can 16 and a region 34 surrounded by the groove portion 32. The annular groove portion 32 may be provided at any position on the outer surface of the bottom of the exterior can 16, but it is desirable that the annular groove portion 32 be provided such that the region 34 surrounded by the groove portion 32 is located at the center of the bottom.

[0019] The annular groove portion 32 is, for example, an imprint formed from the outer surface side of the bottom of the exterior can 16, and the portion of the bottom of the exterior can where the groove portion 32 is formed becomes a thin-walled portion that is thinner than other portions. When the internal pressure of the battery reaches a predetermined pressure, at least a part of the groove portion 32 breaks, and at least a part of the region 34 surrounded by the groove portion 32 opens. Thereby, the gas inside the battery is discharged.

[0020] The plan view shape of the groove 32 is not particularly limited as long as it is annular, and examples include the circular, elliptical, rectangular, and other polygonal shapes shown in Figure 2. Here, annular means not only a closed ring shape that extends continuously around the groove 32 in the direction of extension, but also an open ring shape that extends continuously for less than one full turn in the direction of extension. An example of a groove 32 with an open ring shape in plan view is a C-shape as shown in Figure 4. When the groove 32 is a closed ring shape, the circumferential direction of the groove 32 is the direction of extension. When the groove 32 is an open ring shape, the direction in which the groove 32 extends from one end (indicated by A in Figure 4) to the other end (indicated by B in Figure 4) is the direction of extension. In the case of an open ring-shaped groove 32, the region enclosed by the groove 32 and the imaginary straight line (indicated by α in Figure 4) connecting one end and the other end of the groove 32 becomes the region 34 enclosed by the groove 32. Furthermore, in terms of gas discharge, it is desirable for the distance between one end and the other end of the open-ring groove 32 to be short. That is, it is preferable for the distance of the imaginary straight line connecting one end and the other end of the groove 32 to be short. For example, the distance of the imaginary straight line connecting one end and the other end of the groove 32 is preferably 20% or less of the longest diameter within the region 34 surrounded by the groove 32, more preferably 10% or less, and even more preferably 5% or less. However, the open-ring shape in this disclosure is defined as the case where the distance of the imaginary straight line connecting one end and the other end of the groove 32 is 50% or less of the longest diameter within the region 34 surrounded by the groove 32, and cases exceeding 50% are not included in the open-ring shape in this disclosure.

[0021] As shown in Figure 3, the groove 32 has an inner circumferential wall surface 36, an outer circumferential wall surface 38, and a bottom surface 40 connecting the inner circumferential wall surface 36 and the outer circumferential wall surface 38. The cross-sectional shape of the groove 32 is asymmetrical with respect to an axis C that passes through the center of the bottom surface 40 and extends parallel to the depth direction of the groove 32 (arrow Y in Figure 3). The inner circumferential wall surface 36 is substantially perpendicular to the width direction of the groove 32 (arrow X in Figure 3), and the outer circumferential wall surface 38 is inclined with respect to the depth direction of the groove 32 such that the width of the groove 32 narrows towards the bottom surface 40. Here, the cross-sectional shape of the groove 32 is the cross-sectional shape of the groove 32 in a cross section perpendicular to the extension direction of the groove 32. The width direction of the groove 32 is the direction perpendicular to the extension direction and the depth direction of the groove 32. Furthermore, the depth direction of the groove 32 is the thickness direction of the bottom of the outer can 16 (i.e., the direction from the outer surface 16a to the inner surface 16b of the bottom of the outer can 16). The inner circumferential wall surface 36 being substantially perpendicular to the width direction of the groove 32 is defined as the angle (θ) between the width direction of the groove 32 and the extension direction of the inner circumferential wall surface 36. Ｘ This means that the angle is within the range of 90° ± 5°.

[0022] When the internal pressure of the battery increases, stress is applied to the groove 32, but the stress applied to the groove 32 is more likely to be applied to the inner wall surface 36 than to the outer wall surface 38. In this embodiment, by making the cross-sectional shape of the groove 32 asymmetrical, with the outer wall surface 38 inclined and the inner wall surface 36 being approximately vertical, the stress applied to the groove 32 can be concentrated more on the inner wall surface 36 without being released. As a result, when the internal pressure of the battery increases, high stress is applied to the annular groove 32. When high stress is applied to the annular groove 32 when the internal pressure of the battery increases, it is possible to suppress the increase in the breaking pressure of the annular groove 32 even if the diameter of the groove 32 is increased and the area of ​​the region 34 surrounded by the groove 32 is increased. Furthermore, in this embodiment, the increase in the breaking pressure of the groove 32 can be suppressed even if the groove 32 is not provided within the region 34 surrounded by the annular groove 32. Therefore, the region 34 surrounded by the annular groove 32 can be used as a surface to be processed when the secondary battery 10 is incorporated into the product.

[0023] The inclination angle of the outer peripheral side wall surface 38 with respect to the depth direction of the groove portion 32, that is, the angle (θ) formed by the depth direction of the groove portion 32 and the extending direction of the outer peripheral side wall surface 38 Ｙ ) is preferably 40° or less, and more preferably 30° or less. By setting the inclination angle of the outer peripheral side wall surface 38 to 40° or less, the stress applied to the groove portion 32 can be concentrated on the more inner peripheral side wall surface 36 without releasing it. Therefore, when the internal pressure of the battery rises, a high stress is applied to the annular groove portion 32. Further, the lower limit of the inclination angle of the outer peripheral side wall surface 38 is preferably 5° or more, and more preferably 10° or more, in terms of being able to concentrate the stress applied to the groove portion 32 on the more inner peripheral side wall surface 36. In the case where the outer peripheral side wall surface 38 of the groove portion 32 is curved in the vertical cross-section with respect to the extending direction of the groove portion 32, the extending direction of the outer peripheral side wall surface 38 means the extending direction of the tangent line of the wall surface, and among these, the one taking the largest angle with respect to the depth direction of the groove portion 32 is taken as the above inclination angle.

[0024] The shape of the bottom surface 40 is preferably such that the entire bottom surface 40 is curved, for example, in terms of applying a high stress to the annular groove portion 32 when the internal pressure of the battery rises. Further, the radius of curvature of the curved bottom surface 40 is preferably 0.2 mm or less, and more preferably 0.01 mm or more and 0.1 mm or less, for example, in terms of applying a high stress to the annular groove portion 32 when the internal pressure of the battery rises.

[0025] The width of the groove portion 32 may be, for example, 0.5 mm or more and 3 mm or less, or 0.8 mm or more and 2.5 mm or less. The depth of the groove portion 32 may be, for example, 0.1 times or more and 0.8 times or less, or 0.2 times or more and 0.7 times or less, with respect to the thickness of the bottom of the outer can in the region where the groove portion 32 is not provided.

[0026] Hereinafter, as an experimental example, in the groove portion 32 of the present embodiment and the groove portions (32a, 32b) of the comparison objects, the maximum stress applied to the tip of the groove portion (bottom surface 40) when the internal pressure of the battery rises to a predetermined value was obtained by simulation. FIG. 5 is a model of the groove portion 32 of the example and models of the groove portions (32a, 32b) of the comparative examples used in the simulation.

[0027] Figure 5(A) is a model of the groove portion 32 of the present embodiment. In the model of the groove portion 32 of the present embodiment, the inner peripheral wall surface 36 is perpendicular to the width direction of the groove portion 32, and the outer peripheral wall surface 38 is inclined at 30° with respect to the depth direction of the groove portion 32. That is, the angle (θ Ｘ ) formed between the width direction of the groove portion 32 and the extending direction of the inner peripheral wall surface 36 is 90°, and the angle (θ Ｙ ) formed between the depth direction of the groove portion 32 and the extending direction of the outer peripheral wall surface 38 is 30°. The skirt portion 42a extending from the inner peripheral wall surface 36 to the flat portion of the region 34 surrounded by the groove portion 32 is curved, and the radius of curvature of this portion is 100 mm. Also, the skirt portion 42b extending from the outer peripheral wall surface 38 to the flat portion of the outer surface 16a of the bottom of the outer can 16 is also curved, and the radius of curvature of this portion is 200 mm. The entire bottom surface 40 is curved, and the radius of curvature of the bottom surface 40 is 0.05 mm.

[0028] Figure 5(B) is a model of the groove portion 32a of Comparative Object 1. The cross-sectional shape of the model of the groove portion 32a of Comparative Object 1 is line-symmetric. The angle (θ １ ) formed between the depth direction of the groove portion 32a and the extending direction of the inner peripheral wall surface 36 of the groove portion 32a and the angle (θ ２ ) formed between the depth direction of the groove portion 32a and the extending direction of the outer peripheral wall surface 38 of the groove portion 32a are each 15°. Also, the skirt portions 42a and 42b are curved, and the radius of curvature of these portions is 200 mm. The entire bottom surface 40 is curved, and the radius of curvature of the bottom surface 40 is 0.05 mm.

[0029] Figure 5(C) is a model of the groove portion 32b of Comparative Object 2. In the model of the groove portion 32b of Comparative Object 2, the outer peripheral wall surface 38 is perpendicular to the width direction of the groove portion 32b, and the inner peripheral wall surface 36 is inclined at 30° with respect to the depth direction of the groove portion 32b. That is, the angle (θ ３ ) formed between the width direction of the groove portion 32b and the extending direction of the outer peripheral wall surface 38 is 90°, and the angle (θ １The angle is 30°. The base portions 42a and 42b are curved, with the radius of curvature of base portion 42a being 200 mm and the radius of curvature of base portion 42b being 100 mm. The entire bottom surface 40 is curved, with the radius of curvature of the bottom surface 40 being 0.05 mm.

[0030] The simulation was performed using Hyperworks.

[0031] The simulation results showed that, using the maximum stress at the tip of the groove 32a in the model of comparison target 1 as a baseline, the maximum stress at the tip of the groove 32 in the model of the embodiment increased by approximately 10% compared to the baseline, while the maximum stress at the tip of the groove 32b in the model of comparison target 2 decreased by approximately 10% compared to the baseline. Therefore, it can be said that with the groove 32 of this embodiment, high stress is applied to the annular groove 32 when the internal pressure of the battery increases.

[0032] An example of a positive electrode 11, a negative electrode 12, and a separator 13 is described below.

[0033] The positive electrode 11 comprises a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode current collector can be made of a metal foil that is stable in the positive electrode potential range, such as aluminum, or a film with the metal disposed on its surface. The thickness of the positive electrode current collector is, for example, 10 μm to 30 μm.

[0034] The positive electrode mixture layer may be provided on one side of the positive electrode current collector or on both sides. The thickness of the positive electrode mixture layer is, for example, 10 μm to 150 μm on one side of the positive electrode current collector. The positive electrode mixture layer includes, for example, a positive electrode active material, a conductive agent, and a binder. The positive electrode can be manufactured, for example, by applying a positive electrode mixture slurry containing the positive electrode active material, conductive agent, binder, etc., onto the positive electrode current collector, drying the coating film, and then rolling the coating film using a roller or the like.

[0035] Examples of positive electrode active materials included in the positive electrode mixture layer include lithium transition metal composite oxides containing transition metal elements such as Co, Mn, and Ni. ｘ CoO ２ Li ｘ NiO ２ Liｘ MnO ２ Li ｘ Co ｙ Ni １－ｙ O ２ Li ｘ Co ｙ M １－ｙ O ｚ Li ｘ Ni １－ｙ M ｙ O ｚ Li ｘ Mn ２ O ４ Li ｘ Mn ２－ｙ M ｙ O ４ LiMPO ４ Li ２ MPO ４ F (where M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, or B, with 0 < x ≤ 1.2, 0 < y ≤ 0.9, and 2.0 ≤ z ≤ 2.3). These may be used individually or in combination of multiple elements.

[0036] In order to increase the capacity of the secondary battery 10, it is preferable that the positive electrode active material contains a lithium nickel composite oxide. As for the lithium nickel composite oxide, Li ｘ NiO ２ Li ｘ Co ｙ Ni １－ｙ O ２ Li ｘ Ni １－ｙ M ｙ O ｚ Examples include (where M is at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B, with 0 < x ≤ 1.2, 0 < y ≤ 0.9, and 2.0 ≤ z ≤ 2.3).

[0037] Examples of conductive agents included in the positive electrode mixture layer include acetylene black (AB), carbon black (CB) such as Ketjenblack, carbon nanotubes (CNT), graphene, and carbon-based particles such as graphite. These may be used individually or in combination of two or more types.

[0038] Examples of binders included in the positive electrode mixture layer include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts, and polyvinyl alcohol (PVA). These may be used individually or in combination of two or more types.

[0039] The negative electrode 12 comprises a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector. The negative electrode current collector can be made of a metal foil that is stable in the negative electrode potential range, such as copper, or a film with the metal disposed on its surface. The thickness of the negative electrode current collector is, for example, 5 μm to 30 μm.

[0040] The negative electrode mixture layer may be provided on one side of the negative electrode current collector or on both sides. The thickness of the negative electrode mixture layer is, for example, 10 μm to 150 μm on one side of the negative electrode current collector. The negative electrode mixture layer includes, for example, a negative electrode active material and a binder. The negative electrode can be manufactured, for example, by applying a negative electrode mixture slurry containing the negative electrode active material, binder, etc., onto the negative electrode current collector, drying the coating, and then rolling the coating using a roller or the like.

[0041] The negative electrode active material contained in the negative electrode mixture layer is not particularly limited as long as it can reversibly intercept and release lithium ions, and generally carbon materials such as graphite are used. The graphite may be any of the following: natural graphite such as flake graphite, lump graphite, or clay-like graphite; lump artificial graphite; or artificial graphite such as graphitized mesophase carbon microbeads.

[0042] As the negative electrode active material, metals that alloy with Li, such as Si and Sn, metal compounds containing Si and Sn, lithium titanium composite oxides, etc., may be used. For example, SiO ｘ Si-containing compounds represented by (0.5 ≤ x ≤ 1.6), or Li ２ｙ SiO （２＋ｙ）Examples include Si-containing compounds in which fine Si particles are dispersed in a lithium silicate phase represented by (0 < y < 2).

[0043] The binder included in the negative electrode mixture layer may be one of those exemplified on the positive electrode side. The negative electrode mixture layer may also contain the conductive material mentioned above.

[0044] For the separator 13, for example, a porous sheet having ion permeability and insulating properties can be used. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator include polyethylene, olefin resins such as polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. Alternatively, it may be a multilayer separator containing a polyethylene layer and a polypropylene layer, or a separator with a material such as aramid resin or ceramic coated on its surface may be used.

[0045] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 15 Battery case, 16 Outer casing, 16a Outer surface, 16b Inner surface, 17 Sealing body, 18, 19 Insulating plate, 20 Positive electrode lead, 21 Negative electrode lead, 22 Protruding part, 23 Filter, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 28 Gasket, 30 Safety valve, 32, 32a, 32b Groove part, 34 Region, 36 Inner circumferential wall surface, 38 Outer circumferential wall surface, 40 Bottom surface, 42a, 42b Base part.

Claims

1. A secondary battery comprising an electrode body, an outer casing having an opening and housing the electrode body, and a sealing body for sealing the opening, wherein a safety valve is provided at the bottom of the outer casing, which operates when the internal pressure of the battery reaches a predetermined pressure, the safety valve has an annular groove provided on the outer surface of the bottom of the outer casing, the annular groove has an inner circumferential wall surface, an outer circumferential wall surface, and a bottom surface connecting the inner circumferential wall surface and the outer circumferential wall surface, the inner circumferential wall surface is substantially perpendicular to the width direction of the annular groove, and the outer circumferential wall surface is inclined with respect to the depth direction of the annular groove such that the width of the annular groove narrows towards the bottom surface, the secondary battery.

2. The secondary battery according to claim 1, wherein the entire bottom surface is curved.

3. The secondary battery according to claim 1 or 2, wherein the angle of inclination of the outer peripheral wall surface with respect to the depth direction of the annular groove is 40° or less.

4. The secondary battery according to claim 2, wherein the radius of curvature of the bottom surface is 0.2 mm or less.

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