Patent Publication Number: US-2022238955-A1

Title: Secondary battery, method for manufacturing secondary battery, electronic device, and electric tool

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of PCT patent application no. PCT/JP2020/044024 filed on Nov. 26, 2020, which claims priority to Japanese patent application no. JP2019-226492 filed on Dec. 16, 2019, the entire contents of which are being incorporated herein by reference. 
    
    
     BACKGROUND 
     The present application relates to a secondary battery, a method for manufacturing a secondary battery, an electronic device, and electric tool. 
     Applications of lithium ion batteries have been expanded to electric tools, electric vehicles, and the like. In electronic devices including these large devices, impact is applied from the outside to damage the battery in some cases. Impact resistance of the battery is therefore one of important factors, and various development studies have been conducted. 
     For example, a resistance value of a battery is reduced by laser welding a battery lid and a safety cover of a safety valve mechanism. 
     SUMMARY 
     The present disclosure relates to a secondary battery, a method for manufacturing a secondary battery, an electronic device, and electric tool. 
     However, because a conventional battery uses a laser welding method for welding the battery lid and the safety cover of the safety valve mechanism, there is a problem that the battery is weak against repeating impact and low in impact resistance. 
     The present technology, in an embodiment, is directed to providing a battery having a high vibration resistance. 
     The present technology, in an embodiment, is directed to a secondary battery including an electrode wound body having a structure in which a band-shaped positive electrode and a band-shaped negative electrode are stacked and wound with a separator interposed therebetween, an electrolyte solution, a battery can that accommodates the electrode wound body and the electrolyte solution, a battery lid that closes an open end portion of the battery can, and a safety valve mechanism provided between the battery lid and the electrode wound body, wherein the safety valve mechanism includes at least a safety cover, an outer peripheral portion of the battery lid and an outer peripheral portion of the safety cover are joined, and an area of a region where the outer peripheral portion of the battery lid and the outer peripheral portion of the safety cover are joined is 18.1% or more and 25.0% or less of a sectional area of the battery in a radial direction. 
     The present technology, in an embodiment, is directed to a method for manufacturing a secondary battery, the secondary battery including an electrode wound body having a structure in which a band-shaped positive electrode and a band-shaped negative electrode are stacked and wound with a separator interposed therebetween, an electrolyte solution, a battery can that accommodates the electrode wound body and the electrolyte solution, a battery lid that closes an open end portion of the battery can, and a safety valve mechanism provided between the battery lid and the electrode wound body, wherein the safety valve mechanism includes at least a safety cover, an outer peripheral portion of the battery lid and an outer peripheral portion of the safety cover are joined by a welding method, and an area of a region where the outer peripheral portion of the battery lid and the outer peripheral portion of the safety cover are joined is 18.1% or more and 25.0% or less of a sectional area of the battery in a radial direction. 
     The present technology, in an embodiment, can realize a battery in which the portion where the battery lid and the safety cover are joined is resistant to repeating vibrations. Note that contents of the present technology are not to be construed as being limited by the effects exemplified in the present specification. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic view of a battery according to one embodiment. 
         FIG. 2A  is a view of a relatively large battery in which a safety cover and a battery lid are integrated as viewed from the safety cover side,  FIG. 2B  is a sectional view taken along the line AA′ in  FIG. 2A ,  FIG. 2C  is a view of a relatively small battery in which a safety cover and a battery lid are integrated as viewed from the safety cover side, and  FIG. 2D  is a sectional view taken along the line BB′ in  FIG. 2C . 
         FIG. 3  is a view showing a part of a battery in which a safety cover with a protrusion is arranged. 
         FIG. 4  is a connection diagram used for describing a battery pack as an application example of the present technology. 
         FIG. 5  is a connection diagram used for describing an electric tool as an application example of the present technology. 
         FIG. 6  is a connection diagram used for describing an electric vehicle as an application example of the present technology. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, the present technology will be described with reference to the drawings according to one or more embodiments. 
     Preferred specific examples of the present are described below according to an embodiment, and the content of the present technology is not limited thereby. 
     In the embodiment of the present technology, as a secondary battery, a lithium ion battery having a cylindrical shape will be described as an example. 
     First, the overall configuration of the lithium ion battery will be described according to an embodiment.  FIG. 1  is a schematic sectional view of a lithium ion battery  1 . The lithium ion battery  1  is a cylindrical lithium ion battery  1  in which an electrode wound body  20  is accommodated inside a battery can  11  as shown in  FIG. 1 . 
     Specifically, the lithium ion battery  1  includes a pair of insulators  12  and  13  and an electrode wound body  20  inside the battery can  11  having a cylindrical shape. The lithium ion battery  1  may further include, for example, one kind, or two or more kinds of a positive temperature coefficient (PTC) element, a reinforcing member, and the like inside the battery can  11 . 
     The battery can  11  is a member that mainly accommodates the electrode wound body  20 . The battery can  11  is, for example, a cylindrical container in which one end portion is open and the other end portion is closed. That is, the battery can  11  has an open end portion. The battery can  11  contains, for example, one kind, or two or more kinds of metal materials such as iron, aluminum, and alloys thereof. Note that, for example one kind, or two or more kinds of metal materials such as nickel may be plated on the surface of the battery can  11 . 
     The insulators  12  and  13  are sheet-like members each having a face substantially perpendicular to a winding axis direction (vertical direction of  FIG. 1 ) of the electrode wound body  20 . The insulators  12  and  13  are arranged in such a manner as to sandwich the electrode wound body  20  therebetween. As a material of the insulators  12  and  13 , polyethylene terephthalate (PET), polypropylene (PP), bakelite, or the like is used. Examples of bakelite include paper bakelite and cloth bakelite produced by applying a phenolic resin to paper or cloth and then heating the paper or cloth. 
     A crimp structure  11 R is formed at the open end portion of the battery can  11 , in which a battery lid  14  and a safety valve mechanism  30  are crimped with a gasket  15 . This allows the battery can  11  to be sealed in a state where the electrode wound body  20  and the like are accommodated inside the battery can  11 . 
     The battery lid  14  is a member that closes the open end portion of the battery can  11  in a state where the electrode wound body  20  and the like are accommodated inside the battery can  11 , and is formed from iron plated with nickel. 
     The battery lid  14  contains, for example, the same material as the material for forming the battery can  11 . A central region of the battery lid  14  protrudes in the vertical direction of  FIG. 1 . 
     The gasket  15  is a member that mainly seals a gap between a bent portion  11 P (also referred to as crimp portion) of the battery can  11  and the battery lid  14  by being interposed between the bent portion  11 P and the battery lid  14 . For example, asphalt or the like may be applied to the surface of the gasket  15 . 
     The gasket  15  contains an insulating material. The kind of the insulating material is not particularly limited and is a polymer material such as polybutylene terephthalate (PBT) or polypropylene (PP). This is because the gap between the bent portion  11 P and the battery lid  14  is sufficiently sealed while the battery can  11  and the battery lid  14  are electrically separated from each other. 
     The safety valve mechanism  30  mainly releases the internal pressure of the battery can  11  by releasing the sealed state of the battery can  11  as necessary when the pressure inside the battery can  11  (internal pressure) increases. The cause of the increase in the internal pressure of the battery can  11  is, a gas generated due to a decomposition reaction of an electrolyte solution during charging and discharging. 
     In the safety valve mechanism  30 , a safety cover  31  is a substantially circular plate-like member and is also called a valve body. The safety cover  31  is made of, for example, aluminum. The central portion of the safety cover  31  may have a protrusion protruding toward the electrode wound body  20  as shown in  FIGS. 1 and 2 . The outer peripheral portion of the safety cover  31  is joined to the outer peripheral portion of the battery lid  14  by welding as shown in  FIGS. 2A to 2D . The welding method is, for example, an ultrasonic welding method. A part of a region  32  where the safety cover  31  and the battery lid  14  are joined is covered with the gasket  15  ( FIG. 1 ) and fixed by the battery can  11 . The safety cover  31  and the battery lid  14  have structures as shown in  FIGS. 2A and 2B  when the outer diameter of the battery is relatively large (for example, when the outer diameter is about 18 mm or about 20 mm), and have structures as shown in  FIGS. 2C and 2D  when the outer diameter of the battery is relatively small (for example, when the outer diameter is about 14 mm). The region  32  joined by welding is a hatched region in  FIGS. 2A and 2C . 
     The area of the region  32  where the outer peripheral portion of the battery lid  14  and the outer peripheral portion of the safety cover  31  are joined by welding is preferably equal to or more than a threshold value in order to be resistant to an impact from the outside of the battery  1 , and is preferably equal to or less than a certain value in order to easily release the internal gas when the pressure rises due to generation of the gas inside the battery  1 . As exemplified in the following Examples, the area of the region  32  where the outer peripheral portion of the battery lid  14  and the outer peripheral portion of the safety cover  31  are joined by welding is preferably, for example, 18.5% or more and 25.0% or less of a sectional area of the battery in the radial direction. 
     In the cylindrical lithium ion battery, a band-shaped positive electrode  21  and a band-shaped negative electrode  22  are spirally wound with a separator  23  interposed therebetween, and are accommodated in the battery can  11  in a state of being impregnated with an electrolyte solution. The positive electrode  21  is obtained by forming a positive electrode active material layer on one face or both faces of a positive electrode current collector, and the negative electrode  22  is obtained by forming a negative electrode active material layer on one side or both sides of a negative electrode current collector, which is not shown. The material of the positive electrode current collector is a metal foil containing aluminum or an aluminum alloy. The material of the negative electrode current collector is a metal foil containing nickel, a nickel alloy, copper, or a copper alloy. The separator  23  is a porous insulating film, which enables movement of lithium ions while electrically insulating the positive electrode  21  and the negative electrode  22 . 
     At the center of the electrode wound body  20 , a space (central space  20 C) generated when the positive electrode  21 , the negative electrode  22 , and the separator  23  are wound is provided, and a center pin  24  is inserted into the central space  20 C ( FIG. 1 ). The center pin  24  can be omitted. 
     A positive electrode lead  25  is connected to the positive electrode  21 , and a negative electrode lead  26  is connected to the negative electrode  22  ( FIG. 1 ). The positive electrode lead  25  contains a conductive material such as aluminum. The positive electrode lead  25  is electrically connected to the battery lid  14  via a safety valve mechanism  30 . The negative electrode lead  26  contains a conductive material such as nickel. The negative electrode lead  26  is electrically connected to the battery can  11 . Detailed configurations and materials of the positive electrode  21 , the negative electrode  22 , the separator  23 , and the electrolyte solution will be described later. 
     The positive electrode active material layer contains at least a positive electrode material (positive electrode active material) capable of occluding and releasing lithium, and may further contain a positive electrode binder, a positive electrode conductive agent, and the like. The positive electrode material is preferably a lithium-containing composite oxide or a lithium-containing phosphate compound. The lithium-containing composite oxide has, for example, a layered rock salt-type or spinel-type crystal structure. The lithium-containing phosphate compound has, for example, an olivine type crystal structure. 
     The positive electrode binder contains a synthetic rubber or a polymer compound. Examples of the synthetic rubber include styrene-butadiene-based rubber, fluorine-based rubber, and ethylene propylene diene. Examples of the polymer compound include polyvinylidene fluoride (PVdF) and polyimide. 
     The positive electrode conductive agent is a carbon material such as graphite, carbon black, acetylene black, or Ketjen black. The positive electrode conductive agent may be a metal material or a conductive polymer. 
     The surface of the negative electrode current collector is preferably roughened for improving close contact with the negative electrode active material layer. The negative electrode active material layer contains at least a negative electrode material (negative electrode active material) capable of occluding and releasing lithium, and may further contain a negative electrode binder, a negative electrode conductive agent, and the like. 
     The negative electrode material includes, for example, a carbon material. The carbon material is graphitizable carbon, non-graphitizable carbon, graphite, low crystalline carbon, or amorphous carbon. The shape of the carbon material is a fibrous, spherical, granular, or scaly shape. 
     The negative electrode material contains, for example, a metal-based material. Examples of the metal-based material include Li (lithium), Si (silicon), Sn (tin), Al (aluminum), Zr (zinc), and Ti (titanium). The metal-based element forms a compound, a mixture, or an alloy with another element, and examples thereof include silicon oxide (SiO x  (0&lt;x≤2)), silicon carbide (SiC), an alloy of carbon and silicon, and lithium titanium oxide (LTO). 
     In the lithium ion battery  1 , when the open circuit voltage (that is, the battery voltage) at the time of full charge is 4.25 V or more, the release amount of lithium per unit mass increases as compared with the case where the open circuit voltage at the time of full charge is low with the same positive electrode active material. As a result, a high energy density can be obtained. 
     The separator  23  is a porous film containing a resin, and may be a layered film of two or more kinds of porous films. Examples of the resin include polypropylene and polyethylene. The separator  23  may include a resin layer on one side or both sides of a porous film as a substrate layer. This is because close contact of the separator  23  to each of the positive electrode  21  and the negative electrode  22  improves, which suppresses the distortion of the electrode wound body  20 . 
     The resin layer contains a resin such as PVdF. In the case of forming the resin layer, a solution in which a resin is dissolved in an organic solvent is applied to the substrate layer, and then the substrate layer is dried. The substrate layer may be immersed in the solution and thereafter dried. The resin layer preferably contains inorganic particles or organic particles from the viewpoint of improving the heat resistance and the safety of the battery. The kind of the inorganic particles is aluminum oxide, aluminum nitride, aluminum hydroxide, magnesium hydroxide, boehmite, talc, silica, mica, or the like. In place of the resin layer, a surface layer formed by a sputtering method, an atomic layer deposition (ALD) method, or the like and containing inorganic particles as a main component may be used. 
     The electrolyte solution contains a solvent and an electrolyte salt, and may further contain an additive or the like as necessary. The solvent is a nonaqueous solvent such as an organic solvent, or water. An electrolyte solution containing a nonaqueous solvent is referred to as a nonaqueous electrolyte solution. Examples of the nonaqueous solvent include a cyclic carbonate ester, a chain carbonate ester, a lactone, a chain carboxylate ester, and a nitrile (mononitrile). 
     Lithium salts are typical examples of the electrolyte salt, but a salt other than lithium salts may be contained. Examples of the lithium salt include lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium perchlorate (LiClO 4 ), lithium methanesulfonate (LiCH 3 SO 3 ), lithium trifluoromethanesulfonate (LiCF 3 SO 3 ), and dilithium hexafluorosilicate (Li 2 SF 6 ). These salts can also be used in mixture, and among them, it is preferable to use LiPF 6  and LiBF 4  in mixture from the viewpoint of improving battery characteristics. The content of the electrolyte salt is not particularly limited, and is preferably 0.3 mol/kg to 3 mol/kg with respect to the solvent. 
     Next, a method for manufacturing a secondary battery will be described according to an embodiment. First, in the case of producing the positive electrode  21 , a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent are mixed to produce a positive electrode mixture. Subsequently, the positive electrode mixture is dispersed in an organic solvent to produce a positive electrode mixture slurry in a paste form. Then, the positive electrode mixture slurry is applied to both surfaces of a positive electrode current collector and thereafter dried to form a positive electrode active material layer. Subsequently, the positive electrode active material layer is compression-molded while being heated using a roll press machine to obtain the positive electrode  21 . 
     The negative electrode  22  is produced in the same procedure as the positive electrode  21  described above. 
     Next, the positive electrode lead  25  is connected to the positive electrode current collector and the negative electrode lead  26  is connected to the negative electrode current collector using a welding method. Subsequently, the positive electrode  21  and the negative electrode  22  are stacked with the separator  23  interposed therebetween, and they are wound to form the electrode wound body  20 . Subsequently, the center pin  24  is inserted into the central space  20 C of the electrode wound body  20 . 
     Subsequently, the electrode wound body  20  is accommodated inside the battery can  11  while the electrode wound body  20  is sandwiched between a pair of insulators. Next, one end of the positive electrode lead  25  is connected to the safety valve mechanism  30  and one end of the negative electrode lead  26  is connected to the battery can  11  using a welding method. 
     Subsequently, the battery can  11  is processed using a beading machine (grooving machine) to form a recess in the battery can  11 . Then, an electrolyte solution is injected into the battery can  11  to impregnate the electrode wound body  20  with the electrolyte solution. Subsequently, the outer peripheral portion of the battery lid  14  and the outer peripheral portion of the safety cover  31  of the safety valve mechanism  30  are joined by welding, and the battery lid  14  and the safety valve mechanism  30  are accommodated inside the battery can  11  together with the gasket  15 . 
     Next, as shown in  FIG. 1 , at the open end portion of the battery can  11 , the outer peripheral portion of the battery lid  14  and the outer peripheral portion of the safety cover  31  are welded, and then the battery lid  14  and the safety valve mechanism  30  are crimped with the gasket  15  to form the crimp structure  11 R. Finally, the battery can  11  is closed with the battery lid  14  using a press machine, whereby the secondary battery is completed. 
     EXAMPLES 
     Hereinafter, the present technology will be specifically described based on Examples in which batteries having different welding areas, welding methods, and the like in the region  32  where the battery lid  14  and the safety cover  31  are joined are tested using the secondary battery produced as described above according to an embodiment. The present technology is not limited to Examples described below. 
     The outer peripheral portion of the battery lid  14  and the outer peripheral portion of the safety cover  31  were welded in a ring shape as shown in the hatched region in  FIG. 2A or 2C . The area of the region  32  joined by welding (the area of the region where the outer peripheral portion of the battery lid  14  and the outer peripheral portion of the safety cover  31  are joined by welding) was determined from the inner diameter and the outer diameter of the region  32  joined by welding, and the sectional area of the battery in the radial direction was determined from the outer diameter of the battery (when the outer diameter of the battery was r, the sectional area of the battery in the radial direction was nr 2 /4). Then, a value obtained by dividing the area of the region  32  joined by welding by the sectional area of the battery in the radial direction was defined as a welding rate. In Examples 1 to 6 and Comparative Examples 1 to 7, an ultrasonic welding method was used for welding the outer peripheral portion of the battery lid  14  and the outer peripheral portion of the safety cover  31 . 
     Example 1 
     The welding rate was set to 25.0% by setting the outer diameter of the battery  1  to 13.8 mm, setting the inner diameter of the region  32  joined by welding to 9.52 mm, and setting the outer diameter of the region  32  joined by welding to 11.76 mm. 
     Example 2 
     The welding rate was set to 19.2% by setting the outer diameter of the battery  1  to 13.8 mm, setting the inner diameter of the region  32  joined by welding to 10.09 mm, and setting the outer diameter of the region  32  joined by welding to 11.76 mm. 
     Example 3 
     The welding rate was set to 18.5% by setting the outer diameter of the battery  1  to 13.8 mm, setting the inner diameter of the region  32  joined by welding to 10.15 mm, and setting the outer diameter of the region  32  joined by welding to 11.76 mm. 
     Example 4 
     The welding rate was set to 18.1% by setting the outer diameter of the battery  1  to 13.8 mm, setting the inner diameter of the region  32  joined by welding to 10.19 mm, and setting the outer diameter of the region  32  joined by welding to 11.76 mm. 
     Example 5 
     The welding rate was set to 25.0% by setting the outer diameter of the battery  1  to 18.20 mm, setting the inner diameter of the region  32  joined by welding to 12.79 mm, and setting the outer diameter of the region  32  joined by welding to 15.7 mm. 
     Example 6 
     The welding rate was set to 23.8% by setting the outer diameter of the battery  1  to 18.05 mm, setting the inner diameter of the region  32  joined by welding to 13.00 mm, and setting the outer diameter of the region  32  joined by welding to 15.7 mm. 
     Example 7 
     The welding rate was set to 18.6% by setting the outer diameter of the battery  1  to 18.20 mm, setting the inner diameter of the region  32  joined by welding to 13.60 mm, and setting the outer diameter of the region  32  joined by welding to 15.7 mm. 
     Example 8 
     The welding rate was set to 18.1% by setting the outer diameter of the battery  1  to 18.05 mm, setting the inner diameter of the region  32  joined by welding to 13.69 mm, and setting the outer diameter of the region  32  joined by welding to 15.7 mm. 
     Example 9 
     The welding rate was set to 25.0% by setting the outer diameter of the battery  1  to 21.20 mm, setting the inner diameter of the region  32  joined by welding to 15.10 mm, and setting the outer diameter of the region  32  joined by welding to 18.45 mm. 
     Example 10 
     The welding rate was set to 24.3% by setting the outer diameter of the battery  1  to 21.20 mm, setting the inner diameter of the region  32  joined by welding to 15.20 mm, and setting the outer diameter of the region  32  joined by welding to 18.45 mm. 
     Example 11 
     The welding rate was set to 18.1% by setting the outer diameter of the battery  1  to 21.20 mm, setting the inner diameter of the region  32  joined by welding to 16.10 mm, and setting the outer diameter of the region  32  joined by welding to 18.45 mm. 
     Comparative Example 1 
     The welding rate was set to 25.2% by setting the outer diameter of the battery  1  to 13.8 mm, setting the inner diameter of the region  32  joined by welding to 9.50 mm, and setting the outer diameter of the region  32  joined by welding to 11.76 mm. 
     Comparative Example 2 
     The welding rate was set to 18.0% by setting the outer diameter of the battery  1  to 13.8 mm, setting the inner diameter of the region  32  joined by welding to 10.20 mm, and setting the outer diameter of the region  32  joined by welding to 11.76 mm. 
     Comparative Example 3 
     The welding rate was set to 25.4% by setting the outer diameter of the battery  1  to 18.05 mm, setting the inner diameter of the region  32  joined by welding to 12.80 mm, and setting the outer diameter of the region  32  joined by welding to 15.7 mm. 
     Comparative Example 4 
     The welding rate was set to 18.0% by setting the outer diameter of the battery  1  to 18.05 mm, setting the inner diameter of the region  32  joined by welding to 13.70 mm, and setting the outer diameter of the region  32  joined by welding to 15.7 mm. 
     Comparative Example 5 
     The welding rate was set to 17.8% by setting the outer diameter of the battery  1  to 18.20 mm, setting the inner diameter of the region  32  joined by welding to 13.70 mm, and setting the outer diameter of the region  32  joined by welding to 15.7 mm. 
     Comparative Example 6 
     The welding rate was set to 27.0% by setting the outer diameter of the battery  1  to 21.20 mm, setting the inner diameter of the region  32  joined by welding to 14.80 mm, and setting the outer diameter of the region  32  joined by welding to 18.45 mm. 
     Comparative Example 7 
     The welding rate was set to 15.5% by setting the outer diameter of the battery  1  to 21.20 mm, setting the inner diameter of the region  32  joined by welding to 16.45 mm, and setting the outer diameter of the region  32  joined by welding to 18.45 mm. 
     A vibration test and a combustion test were performed on the above Examples and Comparative Examples after storage at a high temperature and a high humidity. The number of tests is 10 for each test of each example. The vibration test is based on the UN 38.3 standard. A battery having a small change in the internal resistance value (resistance value ACR (mΩ) at an alternating current of 1 kHz) before and after the vibration test (when the rate of increase in the resistance value was 10% or less) was evaluated as OK, and a battery having a large change in the internal resistance value (when the rate of increase in the resistance value was more than 10%) was evaluated as NG. The combustion test is based on the UL 1642 projectile test. The case where the success rate in the combustion test was 90% or more was evaluated as OK, and the case where the success rate in the combustion test was less than 90% was evaluated as NG. In the high temperature and high humidity storage performed before the vibration test and the combustion test, the battery  1  was stored in an environment of a temperature of 60° C. and a humidity of 90% for about 1 month. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Sectional 
                 Inner 
                 Outer 
                   
                   
                   
                   
               
               
                   
                 Outer 
                 area of 
                 diameter 
                 diameter 
               
               
                   
                 diameter 
                 battery 
                 of 
                 of 
                 Area of 
               
               
                   
                 of 
                 in radial 
                 welded 
                 welded 
                 welded 
                 Welding 
               
               
                   
                 battery 
                 direction 
                 portion 
                 portion 
                 portion 
                 rate 
                 Vibration 
                 Combustion 
               
               
                   
                 (mm) 
                 (mm 2 ) 
                 (mm) 
                 (mm) 
                 (mm 2 ) 
                 (%) 
                 test 
                 test 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 13.8 
                 149.50 
                 9.52 
                 11.76 
                 37.42 
                 25.0 
                 OK 
                 OK 
               
               
                 Example 2 
                 13.8 
                 149.50 
                 10.09 
                 11.76 
                 28.64 
                 19.2 
                 OK 
                 OK 
               
               
                 Example 3 
                 13.8 
                 149.50 
                 10.15 
                 11.76 
                 27.69 
                 18.5 
                 OK 
                 OK 
               
               
                 Example 4 
                 13.8 
                 149.50 
                 10.19 
                 11.76 
                 27.05 
                 18.1 
                 OK 
                 OK 
               
               
                 Example 5 
                 18.20 
                 260.02 
                 12.79 
                 15.7 
                 65.08 
                 25.0 
                 OK 
                 OK 
               
               
                 Example 6 
                 18.05 
                 255.75 
                 13.00 
                 15.7 
                 60.83 
                 23.8 
                 OK 
                 OK 
               
               
                 Example 7 
                 18.20 
                 260.02 
                 13.60 
                 15.7 
                 48.30 
                 18.6 
                 OK 
                 OK 
               
               
                 Example 8 
                 18.05 
                 255.75 
                 13.69 
                 15.7 
                 46.37 
                 18.1 
                 OK 
                 OK 
               
               
                 Example 9 
                 21.20 
                 352.81 
                 15.10 
                 18.45 
                 88.23 
                 25.0 
                 OK 
                 OK 
               
               
                 Example 10 
                 21.20 
                 352.81 
                 15.20 
                 18.45 
                 85.85 
                 24.3 
                 OK 
                 OK 
               
               
                 Example 11 
                 21.20 
                 352.81 
                 16.10 
                 18.45 
                 63.74 
                 18.1 
                 OK 
                 OK 
               
               
                 Comparative 
                 13.8 
                 149.50 
                 9.50 
                 11.76 
                 37.72 
                 25.2 
                 OK 
                 NG 
               
               
                 Example 1 
               
               
                 Comparative 
                 13.8 
                 149.50 
                 10.20 
                 11.76 
                 26.89 
                 18.0 
                 NG 
                 OK 
               
               
                 Example 2 
               
               
                 Comparative 
                 18.05 
                 255.75 
                 12.80 
                 15.7 
                 64.88 
                 25.4 
                 OK 
                 NG 
               
               
                 Example 3 
               
               
                 Comparative 
                 18.05 
                 255.75 
                 13.70 
                 15.7 
                 46.16 
                 18.0 
                 NG 
                 OK 
               
               
                 Example 4 
               
               
                 Comparative 
                 18.20 
                 260.02 
                 13.70 
                 15.7 
                 46.16 
                 17.8 
                 NG 
                 OK 
               
               
                 Example 5 
               
               
                 Comparative 
                 21.20 
                 352.81 
                 14.80 
                 18.45 
                 95.27 
                 27.0 
                 OK 
                 NG 
               
               
                 Example 6 
               
               
                 Comparative 
                 21.20 
                 352.81 
                 16.45 
                 18.45 
                 54.79 
                 15.5 
                 NG 
                 OK 
               
               
                 Example 7 
               
               
                   
               
            
           
         
       
     
     When the welding rate was 18.1% (Examples 4, 8, and 11) or more and 24.3% (Examples 1, 5, and 9) or less as a whole, the battery passed the vibration test and the combustion test, and therefore it can be determined that the battery has high impact resistance and heat resistance. For a battery having an outer diameter of about 14 mm, when the welding rate was 18.1% (Example 4) or more and 25.0% (Example 1) or less, the battery passed the vibration test and the combustion test, and therefore it can be determined that the battery has high impact resistance and heat resistance. 
     Next, for the battery having the shape of Example 3, a difference in welding between the outer peripheral portion of the battery lid  14  and the outer peripheral portion of the safety cover  31  due to the welding method was examined. 
     Example 21 
     The welding rate was set to 18.5% by setting the outer diameter of the battery  1  to 13.8 mm, setting the inner diameter of the region  32  joined by welding to 10.15 mm, and setting the outer diameter of the region  32  joined by welding to 11.76 mm. An ultrasonic welding method was used for welding the outer peripheral portion of the battery lid  14  and the outer peripheral portion of the safety cover  31 . As shown in  FIG. 3 , in the outer peripheral portion of the safety cover  31 , a protrusion  41  was formed on the face opposite to the face welded to the battery lid  14 . In the outer peripheral portion of the safety cover  31 , a large number of protrusions  41  were arranged on the whole of the face opposite to the face welded to the battery lid  14 . Each protrusion  41  had a substantially quadrangular pyramid shape with one side of the bottom portion of 50 to 200 μm and a height of 50 to 200 μm. 
     Comparative Example 21 
     The same procedure as in Example 21 was performed except that a laser seam welding method was used for welding the outer peripheral portion of the battery lid  14  and the outer peripheral portion of the safety cover  31 , and no protrusion  41  was formed on the face of the outer peripheral portion of the safety cover  31  opposite to the face welded to the battery lid  14 . 
     Comparative Example 22 
     The same procedure as in Example 21 was performed except that the outer peripheral portion of the battery lid  14  and the outer peripheral portion of the safety cover  31  were not welded and no protrusion  41  was formed on the face of the outer peripheral portion of the safety cover  31  opposite to the face welded to the battery lid  14 . 
     In Example 21 and Comparative Examples 21 and 22, a vibration test was performed after storage at a high temperature and a high humidity under the same conditions as in Examples 1 to 11. The results of the evaluation with the same criteria are shown in Table 2. The number of tests is five for each example. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Welding method 
                 Protrusion 
                 Vibration test 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Example 21 
                 Ultrasonic welding 
                 Present 
                 OK 
               
               
                   
                 method 
               
               
                 Comparative 
                 Laser seam welding 
                 Absent 
                 NG 
               
               
                 Example 21 
                 method 
               
               
                 Comparative 
                 None 
                 Absent 
                 NG 
               
               
                 Example 22 
               
               
                   
               
            
           
         
       
     
     The battery (Example 21) in which the outer peripheral portion of the battery lid  14  and the outer peripheral portion of the safety cover  31  were welded by an ultrasonic welding method passed the vibration test, and therefore it can be determined that the battery has high impact resistance. 
     Next, for the battery having the shape of Example 3, a difference of the presence/absence of the protrusion  41  of the safety cover  31  was examined. 
     Example 31 
     Example 31 is a same example as Example 21. 
     Comparative Example 31 
     The same procedure as in Example 31 was performed except that no protrusion  41  was formed on the face of the outer peripheral portion of the safety cover  31  opposite to the face welded to the battery lid  14 . 
     In Example 31 and Comparative Example 31, a vibration test was performed after storage at a high temperature and a high humidity under the same conditions as in Examples 1 to 11. The results of the evaluation with the same criteria are shown in Table 3. The number of tests is five for each example. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Welding method 
                 Protrusion 
                 Vibration test 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Example 31 
                 Ultrasonic welding 
                 Present 
                 OK 
               
               
                   
                 method 
               
               
                 Comparative 
                 Ultrasonic welding 
                 Absent 
                 NG 
               
               
                 Example 31 
                 method 
               
               
                   
               
            
           
         
       
     
     The battery (Example 31) having the protrusions  41  on the face of the outer peripheral portion of the safety cover  31  opposite to the face welded to the battery lid  14  passed the vibration test, and therefore it can be said that the battery has high impact resistance. 
     Although an embodiment of the present technology has been specifically described above, the content of the present technology is not limited to the above-described embodiment, and various modifications based on the technical idea of the present technology can be made. 
     The outer diameter of the battery is about 14 mm to about 21 mm, but the battery may have a size of outer diameter other than those exemplified. The protrusion  41  may have a shape other than a substantially quadrangular pyramid. The present technology is applicable not only to cylindrical secondary batteries but also to batteries having other shapes as long as the batteries have a battery lid and a safety valve mechanism (safety cover). In this case, the battery may be either a primary battery or a secondary battery. For example, the present technology is applicable to a small button-shaped secondary battery. 
       FIG. 4  is a block diagram showing a circuit configuration example where the secondary battery according to one embodiment or Example of the present technology is applied to a battery pack  300  according to an embodiment. The battery pack  300  includes an assembled battery  301 , a switch unit  304  including a charge control switch  302   a  and a discharge control switch  303   a,  a current detection resistor  307 , a temperature detection element  308 , and a control unit  310 . The control unit  310  controls each device, and can further perform charge and discharge control at the time of abnormal heat generation, and can calculate and correct the remaining capacity of the battery pack  300 . A positive electrode terminal  321  and a negative electrode terminal  322  of the battery pack  300  are connected to a charger or an electronic device, and perform charging and discharging are performed. 
     The assembled battery  301  is formed by connecting a plurality of secondary batteries  301   a  in series and/or in parallel.  FIG. 4  shows, as an example, a case where six secondary batteries  301   a  are connected in 2 parallel 3 series (2P3S). 
     A temperature detection unit  318  is connected to the temperature detection element  308  (for example, a thermistor), measures the temperature of the assembled battery  301  or the battery pack  300 , and supplies the measured temperature to the control unit  310 . A voltage detection unit  311  measures the voltages of the assembled battery  301  and each of the secondary batteries  301   a  constituting the assembled battery, performs A/D conversion on the measured voltages, and supplies the converted voltages to the control unit  310 . A current measurement unit  313  measures current using the current detection resistor  307  and supplies the measured current to the control unit  310 . 
     A switch control unit  314  controls the charge control switch  302   a  and the discharge control switch  303   a  of the switch unit  304  on the basis of the voltage and the current input from the voltage detection unit  311  and the current measurement unit  313 . When the voltage of any of the secondary batteries  301   a  becomes equal to or lower than the overcharge detection voltage (for example, 4.20 V±0.05 V) or the overdischarge detection voltage (2.4 V±0.1 V), the switch control unit  314  sends an OFF control signal to the switch unit  304  to prevent overcharging and overdischarging. 
     After the charge control switch  302   a  or the discharge control switch  303   a  is turned off, charging or discharging can be performed only through a diode  302   b  or a diode  303   b.  As these charge/discharge switches, a semiconductor switch such as a MOSFET can be used. Although the switch unit  304  is provided on the positive side in  FIG. 4 , it may be provided on the negative side. 
     A memory  317  includes a RAM and a ROM, and stores and rewrites the values of the battery characteristics calculated by the control unit  310 , the full charge capacity, the remaining capacity, and the like. 
     The secondary battery according to the embodiment or Example of the present technology described above can be mounted on a device such as an electronic device, an electric transportation device, or a power storage device, and can be used for supplying electric power. 
     Examples of the electronic device include notebook computers, smartphones, tablet terminals, PDAs (personal digital assistants), mobile phones, wearable terminals, digital still cameras, electronic books, music players, game machines, hearing aids, electric tools, televisions, lighting devices, toys, medical devices, and robots. In addition, electric transportation device, a power storage device, an electric tool, and an electric unmanned aerial vehicle to be described later may also be included in the electronic device in a broad sense. 
     Examples of the electric transportation device include electric cars (including hybrid cars), electric motorcycles, electric-assisted bicycles, electric buses, electric carts, automated guided vehicles (AGV), railway vehicles, and the like. The examples also include electric passenger aircrafts and electric unmanned aerial vehicles for transportation. The secondary battery according to the present invention is used not only as a power source for driving these, but also as an auxiliary power supply, a power source for energy regeneration, and the like. 
     Examples of the power storage device include a power storage module for commercial use or household use, and a power storage power source for a building such as a house, a building, or an office, or for a power generation facility. 
     An example of an electric tool, for example, an electric screwdriver to which the present invention can be applied will be schematically described with reference to  FIG. 5 . An electric screwdriver  431  is provided with a motor  433  that transmits rotational power to a shaft  434  and a trigger switch  432  to be operated by a user. A battery pack  430  according to the present invention and a motor control unit  435  are accommodated in a lower housing of a handle of the electric screwdriver  431 . The battery pack  430  is built in or detachable from the electric screwdriver  431 . 
     Each of the battery pack  430  and the motor control unit  435  may be provided with a microcomputer (not shown) so that charge/discharge information of the battery pack  430  can be communicated between them. The motor control unit  435  controls the operation of the motor  433  and can cut off the power supply to the motor  433  at the time of abnormality such as overdischarging. 
     As an example in which the present technology is applied to a power storage system for an electric vehicle,  FIG. 6  schematically shows a configuration example of a hybrid vehicle (HV) employing a series hybrid system. The series hybrid system is a vehicle that travels with an electric power-driving force conversion device using electric power generated by a generator driven by an engine or the electric power temporarily stored in a battery. 
     In a hybrid vehicle  600 , an engine  601 , a generator  602 , and an electric power-driving force conversion device  603  (a DC motor or an AC motor. Hereinafter, it is simply referred to as “motor  603 ”), a driving wheel  604   a,  a driving wheel  604   b,  a wheel  605   a,  a wheel  605   b,  a battery  608 , a vehicle control device  609 , various sensors  610 , and a charging port  611  are mounted. As the battery  608 , the battery pack  300  of the present invention or a power storage module to which a plurality of secondary batteries of the present invention are mounted can be applied. 
     The motor  603  is operated by the electric power of the battery  608 , and the rotational force of the motor  603  is transmitted to the driving wheels  604   a  and  604   b.  The electric power generated by the generator  602  from the rotational force generated by the engine  601  can be stored in the battery  608 . The various sensors  610  control the engine speed through the vehicle control device  609  and control the opening degree of a throttle valve (not shown). 
     When the hybrid vehicle  600  is decelerated by a braking mechanism (not shown), a resistance force at the time of deceleration is applied to the motor  603  as a rotational force, and regenerative electric power generated from the rotational force is stored in the battery  608 . The battery  608  can be charged by being connected to an external power supply via the charging port  611  of the hybrid vehicle  600 . Such an HV vehicle is referred to as a plug-in hybrid vehicle (PHV or PHEV). 
     The secondary battery according to the present technology may also be applied to a downsized primary battery and used as a power source of a pneumatic pressure sensor system (TPMS: Tire Pressure Monitoring system) built in the wheels  604  and  605 . 
     In the above, a series hybrid vehicle has been described as an example, but the present technology can also be applied to a parallel system in which an engine and a motor are used in combination, or a hybrid vehicle in which a series system and a parallel system are combined. Further, the present technology can also be applied to an electric vehicle (EV or BEV) and a fuel cell vehicle (FCV) that travel only by a drive motor without using an engine. 
     DESCRIPTION OF REFERENCE SYMBOLS 
     
         
           1 : Lithium ion battery 
           11 : Battery can 
           12 ,  13 : Insulator 
           20 : Electrode wound body 
           21 : Positive electrode 
           22 : Negative electrode 
           23 : Separator 
           24 : Center pin 
           25 : Positive electrode lead 
           26 : Negative electrode lead 
           31 : Safety cover 
           32 : Region joined by welding 
       
    
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.