Cylindrical secondary battery

Abstract

This cylindrical secondary battery includes an electrode body, an exterior can, a sealing body, and a gasket interposed between the exterior can and the sealing body. The sealing body includes a cap (27) having a ventilation port (27a), and a rupture plate. The cap (27) has an annular flange part (27b) provided on the outer peripheral side, a top plate part (27d) positioned on the battery outer side of the flange part (27b), on the inner peripheral side of the flange part (27b), and a cylindrical protrusion (27c) connecting the flange part (27b) and the top plate part (27d). A groove (27e) is formed in the top plate part (27d).

Description

Cylindrical secondary battery

[0001] The present disclosure relates to a cylindrical secondary battery, and more particularly to a cylindrical secondary battery in which short circuits are suppressed during a flat plate crush test despite an increase in the thickness of the cap of a sealing body.

[0002] Conventionally, cylindrical secondary batteries have been known that include an electrode assembly, a cylindrical outer can with a bottom that houses the electrode assembly, a sealing body that closes the opening of the outer can, and an annular gasket interposed between the outer can and the sealing body. The sealing body may include a cap that can be used as an external terminal.

[0003] Patent Document 1 describes a sealing body cap for a cylindrical battery that has a protrusion formed in the center of a metal plate so as to protrude outside the battery, and a flange formed around the protrusion. The sealing body cap is made of a material that is relatively strong among the components of the sealing body. Therefore, the sealing body incorporates a valve that can be broken by an increase in internal battery pressure together with the sealing body cap, and the sealing body cap has a vent hole.

[0004] A flat plate crush test is conducted on cylindrical secondary batteries as a safety evaluation test, in which a compressive load is applied to the cylindrical secondary battery by two flat plates sandwiched between the side surfaces of the cylindrical secondary battery so as to face each other.

[0005] Japanese Patent Application Laid-Open No. 2020-140866

[0006] As described above, cylindrical secondary batteries including a sealing body with a cap having a vent hole are becoming larger in diameter to achieve higher capacity. As the diameter increases, it is necessary to ensure the strength of the sealing body, so increasing the thickness of the cap is considered. However, if the thickness of the cap's top plate is increased, the cap becomes less likely to deform during a flat plate crush test. As a result, during a flat plate crush test, the outer edge of the flange portion of the cap may break through the gasket and come into contact with the outer can, causing a short circuit. A short circuit is undesirable because it can cause heat generation in the secondary battery.

[0007] Therefore, an object of the present disclosure is to provide a cylindrical secondary battery that can suppress short circuits during a flat plate crush test, regardless of an increase in the thickness of the sealing cap.

[0008] The cylindrical secondary battery according to the present disclosure comprises an electrode assembly in which a first electrode and a second electrode having a polarity different from that of the first electrode are wound with a separator interposed therebetween, a bottomed cylindrical outer can that houses the electrode assembly, a sealing body that closes the opening of the outer can, and an annular gasket that is interposed between the outer can and the sealing body, wherein the sealing body includes a cap having an air vent and a rupture plate that ruptures when the internal pressure of the battery reaches a predetermined value, and the cap has an annular flange portion provided on the outer periphery, a top plate portion that is located on the inner periphery of the flange portion and toward the outside of the battery than the flange portion, and a cylindrical protrusion that connects the flange portion and the top plate portion, and a groove is formed in the top plate portion.

[0009] In the cylindrical secondary battery according to the present disclosure, regardless of an increase in the thickness of the cap of the sealing body, when pressure is applied to the cap from the outside in the radial direction in a flat plate crush test, the grooves promote deformation of the top plate, thereby preventing a short circuit caused by the outer peripheral edge of the cap breaking through the gasket.

[0010] Fig. 2 is an axial cross-sectional view of a cylindrical secondary battery according to an embodiment of the present disclosure. Fig. 3 is a perspective view of the cap shown in Fig. 1. Fig. 4 is a cross-sectional view of the cap shown in Fig. 1. Fig. 5 is a view showing the surface of the top plate portion of the cap shown in Fig. 1 on the inner side of the battery. Fig. 6 is a view corresponding to Fig. 4 in another example of an embodiment. Fig. 7 is a view corresponding to Fig. 4 in another example of an embodiment.

[0011] Hereinafter, an embodiment of a cylindrical secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that the cylindrical secondary battery according to the present disclosure may be a battery using an aqueous electrolyte or a battery using a non-aqueous electrolyte. Below, a secondary battery (lithium ion battery) using a non-aqueous electrolyte will be exemplified as a cylindrical secondary battery according to one embodiment.

[0012] It is anticipated from the beginning that new embodiments may be constructed by appropriately combining the features of the embodiments and variations described below. In the following embodiments, the same components are denoted by the same reference numerals in the drawings, and redundant explanations are omitted. Furthermore, multiple drawings include schematic diagrams, and the dimensional ratios of the length, width, height, etc. of each component between different drawings do not necessarily match. Furthermore, among the components described below, components that are not recited in the independent claims representing the highest concept are optional components and are not essential components. Furthermore, the present disclosure is not limited to the following embodiments and their variations, and various improvements and modifications are possible within the scope of the claims of this application and their equivalents.

[0013] Fig. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 according to an embodiment. Fig. 2 is a perspective view of a cap 27 shown in Fig. 1. Fig. 3 is a cross-sectional view of the cap 27 shown in Fig. 1. As shown in Fig. 1, the cylindrical secondary battery 10 includes a positive electrode 11, a negative electrode 12, and a separator 13, and is provided with an electrode assembly 14 in which the positive electrode 11 and the negative electrode 12 are wound with the separator 13 interposed therebetween. In this example, the positive electrode 11 corresponds to a first electrode, and the negative electrode 12 corresponds to a second electrode having a polarity different from that of the first electrode.

[0014] The cylindrical secondary battery 10 also includes a cylindrical outer can 16 with a bottom that houses the electrode assembly 14, and a sealing body 17 that closes the opening of the outer can 16. The outer can 16 houses the electrode assembly 14 and a nonaqueous electrolyte solution.

[0015] The exterior can 16 is a cylindrical metal container with a bottom, and has a tubular portion 30 and a bottom portion 31 provided at one axial end of the tubular portion 30. The exterior can 16 has a grooved portion 34 (described below) formed on the other axial end, which is the open end of the tubular portion 30, and the sealing body 17 is supported by the grooved portion 34 to close the opening of the exterior can 16. For ease of explanation, the sealing body 17 side of the cylindrical secondary battery 10 will be referred to as the top, and the bottom 31 side of the exterior can 16 will be referred to as the bottom.

[0016] The non-aqueous electrolyte has ion conductivity (e.g., lithium ion conductivity). The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte is not limited to a liquid electrolyte (nonaqueous electrolytic solution), and may be a solid electrolyte using a gel polymer or the like. The cylindrical secondary battery 10 is preferably a lithium ion battery. The electrolyte salt may be, for example, LiBF ４ , LiPF ６ Examples of the non-aqueous solvent include esters such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and methyl propionate (MP), ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0017] Examples of halogen-substituted compounds include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP). From the viewpoint of suppressing a decrease in the charge-discharge cycle characteristics of a cylindrical secondary battery or improving input characteristics, the nonaqueous electrolyte preferably contains 5% by mass or more of FEC, and more preferably 5% by mass to 15% by mass of FEC, based on the mass of the nonaqueous electrolyte.

[0018] Examples of the solid electrolyte include solid or gel polymer electrolytes and inorganic solid electrolytes. 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. Examples of the matrix polymer include a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin. Examples of the inorganic solid electrolyte include materials known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.).

[0019] The electrode assembly 14 has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The positive electrode 11, the negative electrode 12, and the separator 13 are all long strips that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer in both the longitudinal and lateral directions than the positive electrode 11. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and for example, two separators 13 are arranged to sandwich the positive electrode 11.

[0020] A positive electrode tab 20 and a negative electrode tab 21 are connected to the electrode body 14. The positive electrode tab 20 electrically connects the positive electrode 11 and the sealing body 17. The positive electrode tab 20 is provided in the longitudinal center of the positive electrode 11, at a position away from the winding start end and winding end end of the electrode body 14.

[0021] The negative electrode tab 21 is joined to an exposed core portion provided at the winding-start end, which is one longitudinal end of the negative electrode 12 located on the winding-start side of the negative electrode 12. In the example shown in Fig. 1 , the positive electrode tab 20 passes through the opening of the upper insulating plate 18, extends toward the sealing body 17, and is joined to the underside of the sealing body 17, with the sealing body 17 serving as the positive electrode terminal. The negative electrode tab 21 passes through the through-hole of the annular lower insulating plate 19, is bent so as to fit along the inner surface of the bottom 31 of the outer can 16, and is connected to the inner bottom surface of the outer can 16 by welding or the like, with the outer can 16 serving as the negative electrode terminal.

[0022] The negative electrode 12 is disposed on the outermost peripheral surface of the electrode body 14, and the exposed surface of a negative electrode core (described below) constituting the negative electrode 12 abuts against the inner peripheral surface of the outer can 16. This electrically connects both longitudinal ends of the negative electrode 12 to the outer can 16, ensuring good current collection.

[0023] The positive electrode 11 includes a positive electrode core and a positive electrode mixture layer formed on both sides of the core. The positive electrode core can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on the surface. The positive electrode mixture layer preferably contains a positive electrode active material, a conductive agent such as acetylene black, and a binder such as polyvinylidene fluoride (PVdF), and is formed on both sides of the positive electrode core. The thickness of the positive electrode mixture layer is, for example, 40 μm or more and 100 μm or less. The positive electrode active material can be, for example, a lithium transition metal composite oxide containing Ni, Co, Mn, Al, or the like. The positive electrode tab 20 is preferably directly bonded to the positive electrode core by ultrasonic welding or the like.

[0024] The negative electrode 12 has a negative electrode core and a negative electrode mixture layer formed on both sides of the negative electrode core. The negative electrode core can be made of a foil of a metal, such as copper or a copper alloy, that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface. The negative electrode mixture layer contains a negative electrode active material and a binder such as styrene-butadiene rubber (SBR). The thickness of the negative electrode mixture layer is, for example, 40 μm or more and 100 μm or less. For example, graphite, a Si-containing material, or the like is used as the negative electrode active material. The negative electrode tab 21 is preferably directly bonded to the negative electrode core by ultrasonic welding or the like.

[0025] An annular gasket 28 is interposed between the exterior can 16 and the sealing body 17. The sealing body 17 is fixed by crimping to the upper end portion, which is the open end portion of the exterior can 16, via the gasket 28. Specifically, a shoulder portion 38 is formed at the upper end portion of the exterior can 16 by bending the entire periphery radially inward, and the upper end portion of the exterior can 16 is crimped to the peripheral edge of the sealing body 17 via the gasket 28. This seals the inside of the battery.

[0026] Furthermore, the exterior can 16 has a groove 34 formed, for example, by pressing the side surface from the outside, that supports the sealing body 17. The groove 34 is preferably formed in an annular shape along the circumferential direction of the exterior can 16, and supports the sealing body 17 on its upper surface.

[0027] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each component constituting the sealing body 17 has, for example, a disk or ring shape, and all components except the insulating member 25 are electrically connected to each other. The cap 27 has an annular flange portion 27b provided on the outer periphery, a top plate portion 27d located on the inner periphery of the flange portion 27b, toward the outside of the battery (axially above the flange portion 27b), and a cylindrical protrusion 27c connecting the flange portion 27b and the top plate portion 27d. Thus, the cap 27 has a hat-like shape. The top plate portion 27d has a flat disk shape. Ventilation holes 27a are formed in the protrusion portion 27c at multiple positions along the circumferential direction. Some of the ventilation holes 27a may be formed in the top plate portion 27d. The cap 27 is made of a metal material containing iron as a main component, such as steel or stainless steel.

[0028] The internal terminal plate 23 is disk-shaped with a recessed center, and has multiple openings 23a that penetrate vertically. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective centers, with an insulating member 25 interposed between their respective peripheral edges. The positive electrode tab 20 is electrically connected to the center of the lower surface of the internal terminal plate 23. This electrically connects the positive electrode 11 to a member of the sealing body 17 that is closer to the electrode body 14 than the cap 27. The outer peripheral portion of the upper valve body 26 is connected to the lower surface of the flange portion 27b of the cap 27.

[0029] When the internal pressure of the battery increases and reaches a predetermined value, the lower valve body 24 deforms and pushes the upper valve body 26 toward the cap 27, causing it to break, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases and reaches a predetermined value, the upper valve body 26 breaks, causing gas to be discharged through a vent hole 27a provided in the protrusion 27c of the cap 27. In this example, the lower valve body 24 and the upper valve body 26 correspond to a rupture plate. In this example, multiple vent holes 27a are provided in the cap 27, but only one vent hole may be provided in the cap.

[0030] Lower valve body 24 and upper valve body 26 may each be formed with an easily breakable portion, which is a thin, annular or C-shaped portion. Also, only one rupture plate may be provided between internal terminal plate 23 and cap 27. Furthermore, the internal terminal plate may be omitted, and positive electrode tab 20 may be electrically connected to the underside of that one rupture plate.

[0031] When configuring a battery module including a plurality of cylindrical secondary batteries 10, the top plate portion 27d is connected to an external lead 70. When the top plate portion 27d of the cap 27 is used as a terminal portion, the external lead 70 can be easily connected to the terminal portion.

[0032] In cylindrical secondary batteries 10 including a sealing body 17 having a cap 27 with a vent hole 27 a formed therein as described above, the diameter of the cylindrical secondary battery 10 has been increasing in recent years in order to achieve higher capacity, and the outer diameter of the cylindrical secondary battery 10 has tended to increase. As the diameter of the cylindrical secondary battery 10 increases, it is conceivable to increase the thickness of the cap 27 to ensure the strength of the sealing body 17. However, in this case, when pressure is applied to the cap 27 from the radial outside during a flat plate crush test, the cap 27 is less likely to deform. Therefore, during the flat plate crush test, the outer peripheral edge of the flange portion 27 b of the cap 27 may break through the gasket 28 and come into contact with the inner surface of the outer can 16, which serves as the negative terminal, potentially causing a short circuit.

[0033] In this embodiment, in order to solve the above-mentioned problems, a groove is formed as an easily deformable portion in the top plate portion 27d of the cap 27, as will be described below.

[0034] FIG. 4 is a diagram showing the surface of the top plate portion 27d of the cap 27 shown in FIG. 1 on the inner side of the battery (the axially lower side). As shown in FIGS. 3 and 4 , a groove 27e is formed on the surface of the top plate portion 27d on the inner side of the battery. The groove 27e has a planar shape including six straight line portions 27f extending radially outward from the center O of the surface of the top plate portion 27d on the inner side of the battery, and the six straight line portions 27f are connected at a central portion including the center O. Of the six straight line portions 27f, the angle formed by any two adjacent straight line portions 27f in the circumferential direction of the top plate portion 27d is constant. As a result, the six straight line portions 27f include three pairs of straight line portions 27f, each pair consisting of two straight line portions 27f on the same line that are positioned 180 degrees out of phase with each other.

[0035] The depth d1 of groove 27e can be, for example, 5% or more and 70% of the maximum thickness dm of top plate portion 27d. This allows groove 27e to fully exert its effect of promoting deformation of top plate portion 27d while ensuring sufficient strength of sealing body 17. If depth d1 of groove 27e exceeds 70% of the maximum thickness dm, rigidity will be significantly reduced. Furthermore, the length of groove 27e is preferably 10% or more of the diameter of top plate portion 27d.

[0036] According to the cylindrical secondary battery 10 of the embodiment, a groove 27e serving as an easily deformable portion is formed on the inner surface of the battery of the top plate portion 27d of the cap 27 of the sealing body 17. As a result, regardless of an increase in the thickness of the cap 27, when pressure is applied to the cap 27 from the outside in the radial direction in a flat plate crush test, the groove 27e promotes deformation of the top plate portion 27d. This makes it easier for the flange portion 27b to tilt with respect to the axial direction, thereby suppressing a short circuit caused by the outer peripheral edge of the cap 27 breaking through the gasket 28.

[0037] Furthermore, since the groove 27e includes three or more linear portions 27f extending radially outward from the center O of the surface of the top plate portion 27d, even when pressure is applied radially inward from any direction on the outer periphery of the top plate portion 27d, the direction of the applied pressure is offset from the direction in which at least one linear portion 27f extends. This makes the top plate portion 27d more likely to deform in the axial direction, etc. Therefore, during a flat plate crush test, regardless of the circumferential position on the outer periphery of the cylindrical secondary battery 10 with which the flat plate comes into contact, a short circuit caused by the cap 27 breaking through the gasket 28 can be suppressed.

[0038] Although the grooves 27e may be formed on the surface of the top plate 27d facing outward from the battery, they are preferably formed on the surface of the top plate 27d facing inward from the battery, as in this example. This prevents the bonding between the external lead 70 and the top plate 27d from being affected by the grooves 27e, unlike when the grooves 27e are formed on the surface of the top plate 27d facing outward from the battery, making it easier to ensure the bonding strength between the external lead 70 and the top plate 27d.

[0039] 3 shows a configuration in which the overall thickness of flange portion 27b of cap 27 is approximately the same as the maximum thickness of top plate portion 27d. However, the cap is not limited to this configuration, and the outer peripheral portion of the cap may be configured to be thinner than the inner peripheral portion by being pressurized from both axial sides to improve strength.

[0040] It is also possible to form a groove in the flange of the cap to facilitate deformation of the cap. However, to promote deformation of the cap while ensuring the strength of the sealing body, it is preferable to form the groove in the top plate. It is preferable that the groove is not formed in the flange, but a groove may be formed in the flange as long as the strength of the sealing body can be ensured.

[0041] 5 is a diagram corresponding to FIG. 4 of another example of the embodiment. In the configuration of this example, the surface of the top plate portion 27d1 facing inward from the battery includes eight groove-shaped straight portions 27f1 extending radially outward from the center O of the surface of the top plate portion 27d1 facing inward from the battery, and the eight straight portions 27f1 are connected at a central portion including the center O to form the groove 27e1. The angles formed by two adjacent straight portions 27f1 in the circumferential direction of the top plate portion 27d1 are constant for all of the eight straight portions 27f1. As a result, the eight straight portions 27f1 are arranged at positions that are 180 degrees out of phase with each other, and include four pairs of straight portions 27f1, each pair consisting of two straight portions 27f1 positioned on the same line.

[0042] 1 to 4, the number of straight line portions 27f1 of groove 27e1 is increased, and thus, when pressure is applied to cap 27 from the radially outer side in a flat plate crushing test, groove 27e1 further promotes deformation of top plate portion 27d1. This makes it easier for flange portion 27b (see FIG. 3) to tilt with respect to the axial direction, further suppressing a short circuit caused by cap 27 breaking through gasket 28. In this example, the other configurations and operations are the same as those of the configurations in FIGS. 1 to 4.

[0043] 1 to 5, the shape of the groove formed on the surface of the top plate portion of the cap facing inward from the battery may be a shape including three or more straight line portions extending radially outward from the center O of the top plate portion. For example, the groove in the top plate may be a shape including three, four, five, seven, or nine or more straight line portions extending radially outward from the center O, and the angle formed between two adjacent straight line portions in the circumferential direction of the top plate portion 27d may be a constant shape for all adjacent straight line portions.

[0044] 6 is a view corresponding to FIG. 4 of another example of the embodiment. In the configuration of this example, a groove 27g having an annular planar shape including the center O of the surface of the top plate 27d2 on the inner side of the battery is formed on the surface of the top plate 27d2 on the inner side of the battery. More specifically, the groove 27g is a hexagon, which is a polygon that is tangent to the inside of a circle 72 whose center coincides with the center O of the top plate 27d2.

[0045] According to the configuration of this example, even if pressure is applied to the top plate portion 27d2 from any direction on the outer periphery toward the inside in the radial direction during a flat plate crush test, the direction of the applied pressure is different from the direction in which the six straight portions 27h forming the grooves 27g extend, so that the top plate portion 27d2 is likely to deform in the axial direction, etc. As a result, it is possible to suppress a short circuit caused by the outer peripheral edge of the cap 27 breaking through the gasket 28, regardless of the circumferential position of the outer peripheral surface of the cylindrical secondary battery 10 that contacts the flat plate during a flat plate crush test.

[0046] The shape of the annular groove formed on the side surface of the top plate may be other polygonal shapes that are tangent to the inside of a circle whose center coincides with the center of the top plate, such as a triangle, a square, a pentagon, or an octagon.The shape of the annular groove formed on the side surface of the top plate may also be a circle whose center coincides with the center of the top plate.

[0047] Furthermore, the grooves formed on the side surface of the top plate are not limited to a continuous shape, but may be grooves consisting of multiple separated straight lines or multiple curved lines. For example, multiple separated straight or curved grooves may be formed along a circular polygon or circle.

[0048] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0049] Example 1 [Preparation of Positive Electrode] As the positive electrode active material, LiNi ０．８８ Co ０．０９ Al ０．０３ O ２Aluminum-containing lithium nickel cobalt oxide represented by the formula (I) was used. 100 parts by mass of the positive electrode active material, 1 part by mass of acetylene black (AB), and 0.9 parts by mass of polyvinylidene fluoride (PVDF) were mixed, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both sides of a strip-shaped positive electrode current collector made of aluminum foil with a thickness of 15 μm, so that a positive electrode exposed portion was formed. This coating was dried in a dryer at a temperature of 100 to 150 °C, and the NMP was removed, followed by rolling using a roll press. The resulting electrode was then cut to a predetermined electrode plate size to produce a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode current collector. The positive electrode had a thickness of 0.144 mm, a width of 62.6 mm, and a length of 861 mm. An aluminum positive electrode tab was then welded to the positive electrode exposed portion.

[0050] [Preparation of Negative Electrode] 95 parts by mass of graphite powder and 5 parts by mass of Si oxide (SiO ) were mixed, followed by mixing 1 part by mass of sodium carboxymethyl cellulose (CMC-Na) and 1 part by mass of styrene butadiene rubber (SBR), and an appropriate amount of water was added to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both sides of a strip-shaped negative electrode current collector made of copper foil with a thickness of 8 μm, dried, and then rolled using a roller. Thereafter, it was cut to a predetermined electrode plate size to prepare a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode current collector. The negative electrode had a thickness of 0.160 mm, a width of 64.2 mm, and a length of 959 mm. Thereafter, a negative electrode exposed portion where no mixture layer was present and the current collector surface was exposed was provided at the end of the winding start side of the negative electrode, and a nickel negative electrode tab was welded to the negative electrode exposed portion.

[0051] [Preparation of non-aqueous electrolyte] 5 parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a mixed solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1:3, to prepare a non-aqueous electrolyte solution containing lithium hexafluorophosphate (LiPF ６ A non-aqueous electrolyte was prepared by dissolving 1.5 mol / L of ammonium hydroxide in water.

[0052] 1 , the internal terminal plate 23, the lower valve body 24, the insulating member 25, the upper valve body 26, and the cap 27 were stacked in the vertical direction, and the contact portions of the internal terminal plate, the lower valve body 24, the upper valve body 26, and the cap 27 were joined together by welding to form the sealing body 17. At this time, as shown in FIGS. 3 and 4 , a groove 27e including six straight portions extending radially outward from the center of the surface of the top plate portion 27d on the inner side of the battery was formed on the surface of the top plate portion 27d on the inner side of the battery.

[0053] [Secondary Battery Fabrication] A wound electrode assembly was fabricated by spirally winding a positive electrode and a negative electrode with a polyethylene microporous membrane separator interposed therebetween. Insulating plates were placed on the top and bottom of this electrode assembly, and the electrode assembly was housed in an outer can. The negative electrode tab was welded to the bottom of a cylindrical outer can with a bottom, and the positive electrode tab was welded to the lower surface of the internal terminal plate of the sealing body. After injecting a nonaqueous electrolyte into the outer can, the opening of the outer can was sealed with a sealing body via a gasket, producing a cylindrical secondary battery of Example 1. The cylindrical secondary battery had an outer diameter of 20 mm and an axial length of 65 mm.

[0054] Example 2 In fabricating the sealing body, grooves 27e including eight linear portions extending radially outward from the center of the surface of top plate 27d on the inner side of the battery were formed on the surface of top plate 27d of the cap on the inner side of the battery, as shown in Figure 5. A secondary battery was fabricated in the same manner as in Example 1 except for the above.

[0055] Comparative Example A secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the sealing body, both surfaces of the top plate portion 27d of the cap were flat and did not have the grooves 27e.

[0056] [Flat Plate Crush Test] Five test specimens of each of the secondary batteries of Examples 1 and 2 and the Comparative Example were fully charged to a battery voltage of 4.2 V and subjected to a flat plate crush test under the conditions required for the PSE safety certification test. Specifically, a charged secondary battery was placed between two flat plates with the outer periphery of the secondary battery's outer can facing each other. A crushing device was used to apply pressure from the outside of the two flat plates, gradually increasing the applied pressure up to 13 kN. The pressure of the crushing device was released and the test was terminated when a sudden voltage drop occurred until the secondary battery voltage reached 1.4 V, i.e., one-third of the voltage at the start of the test, or when the maximum pressure was applied to the secondary battery. The secondary battery was then checked for the occurrence of a short circuit during the test.

[0057] [Test Results] Table 1 shows the rate of short circuit occurrence during testing of the secondary batteries of Examples 1 and 2 and the comparative example.

[0058]

[0059] From the results shown in Table 1, it can be seen that in the Comparative Example, short circuits occurred in four of the five test samples, whereas in each of Examples 1 and 2, no short circuits occurred in any of the five test samples. The reason for this is thought to be that in the Comparative Example, no groove was formed on the surface of the top plate of the cap, which was highly rigid, causing the flange of the cap to break through the gasket and come into contact with the inner surface of the exterior can, resulting in a short circuit. On the other hand, in each of Examples 1 and 2, groove 27e was formed on the surface of top plate 27d of cap 27 facing the inside of the battery, making it more easily deformable. This meant that flange 27b of cap 27 was inclined relative to the axial direction and did not break through gasket 28, preventing flange 27b from coming into contact with the inner surface of exterior can 16, and therefore no short circuit occurred.

[0060] 10 Cylindrical secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Separator, 14 Electrode body, 16 Outer can, 17 Sealing body, 18 Upper insulating plate, 19 Lower insulating plate, 20 Positive electrode tab, 21 Negative electrode tab, 23 Internal terminal plate, 24 Lower valve body, 25 Insulating member, 26 Upper valve body, 27 Cap, 27a Vent, 27b Flange portion, 27c Protrusion, 27d Top plate portion, 27e Groove, 27f Straight portion, 27g Groove, 28 Gasket, 30 Cylindrical portion, 31 Bottom portion, 34 Grooved portion, 38 Shoulder portion, 70 External lead, 72 Circle.

Claims

1. A cylindrical secondary battery comprising: an electrode assembly in which a first electrode and a second electrode having a polarity opposite to that of the first electrode are wound with a separator interposed therebetween; a cylindrical outer can with a bottom that houses the electrode assembly; a sealing body that closes the opening of the outer can; and an annular gasket interposed between the outer can and the sealing body, wherein the sealing body includes a cap with an air vent and a rupture plate that ruptures when the internal pressure of the battery reaches a predetermined value, and the cap has an annular flange portion provided on the outer periphery, a top plate portion that is located on the inner periphery of the flange portion and toward the outside of the battery than the flange portion, and a cylindrical protrusion that connects the flange portion and the top plate portion, and a groove is formed in the top plate portion.

2. The cylindrical secondary battery according to claim 1, wherein the groove has a planar shape including a straight portion extending radially outward from the center of the cap.

3. The cylindrical secondary battery according to claim 1, wherein the groove has a circular planar shape that includes the center of the cap.

4. The cylindrical secondary battery according to claim 1, wherein the groove includes three or more straight portions extending radially outward from the center of the cap.

5. The cylindrical secondary battery according to claim 1, wherein the groove is a polygon tangent to the inside of a circle having a center coincident with the center of the cap.

6. The cylindrical secondary battery according to any one of claims 1 to 5, wherein the groove is formed on the surface of the top plate portion facing inward from the battery.

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