Patent Publication Number: US-2020280027-A1

Title: Cylindrical nonaqueous electrolyte secondary battery

Description:
TECHNICAL FIELD 
     The present disclosure relates to a cylindrical nonaqueous electrolyte secondary battery. 
     BACKGROUND ART 
     Heretofore, in order to prevent short circuit caused by contact between a positive electrode lead and an electrode group in a cylindrical secondary battery which uses a positive electrode plate provided with a positive electrode lead, an upper insulating plate having an opening portion has been disposed on the electrode group. The opening portion is used to discharge a high pressure gas generated in the secondary battery through the upper insulating plate or to charge an electrolyte liquid to an electrode group side. 
     In order to prevent the short circuit described above, Patent Document 1 has disclosed that the diameter of a through-hole provided for liquid charge at a center of an upper insulating plate is formed smaller than the width of a positive electrode lead. 
     In association with an increase in capacity of a secondary battery, in order to improve discharge performance of gas generated in a secondary battery, Patent Document 2 has disclosed that an opening portion of an upper insulating plate is positively used. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: Japanese Published Unexamined Patent Application No. 3-134955 
     Patent Document 2: International Publication No. 2014/006883 
     SUMMARY OF INVENTION 
     Technical Problem 
     An upper insulating plate not only has a function to secure insulation between an electrode group and a positive electrode lead but also has an important function to control gas emission during internal gas generation in a battery, and to prevent short circuit and to secure emission performance are in a trade-off relationship. In the upper insulating plate, when an opening portion is formed at a side opposite to a lead hole through which the positive electrode lead penetrates with respect to a central axis of the battery, the emission performance can be enhanced. However, when the opening portion is formed, a curved section of the positive electrode lead formed at an upper side than the upper insulating plate is liable to cause short circuit by contact with the electrode group through the opening portion. 
     The present disclosure aims to provide a cylindrical nonaqueous electrolyte secondary battery which can effectively prevent short circuit between an electrode group and a positive electrode lead while discharge performance of internal gas is secured. 
     Solution to Problem 
     A cylindrical nonaqueous electrolyte secondary battery according to the present disclosure is a cylindrical nonaqueous electrolyte secondary battery which comprises: an exterior package can; a sealing body sealing one end of the exterior package can; an electrode group disposed in the exterior package can; and an insulating plate disposed between the sealing body and the electrode group. In the secondary battery described above, the insulating plate has a lead hole through which a positive electrode lead extending from the electrode group penetrates and an opening portion provided at a side opposite to the lead hole with respect to a central axis of the battery orthogonal to the sealing body, the positive electrode lead has a first curved section adjacent to the lead hole and a second curved section provided at a side opposite to the first curved section with respect to the central axis, and when a distance from the central axis to a portion of the second curved section farthest from the central axis is represented by L 1 , and a distance from the central axis to a part of the opening portion nearest to the central axis is represented by L 2 , L 2 &gt;L 1  is satisfied. 
     Advantageous Effects of Invention 
     According to the cylindrical nonaqueous electrolyte secondary battery of the present disclosure, while the discharge performance of internal gas is secured, the short circuit between the electrode group and the positive electrode lead can be effectively prevented. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a cylindrical nonaqueous electrolyte secondary battery according to one example of an embodiment. 
         FIG. 2  is an enlarged view of an A portion of  FIG. 1 . 
         FIG. 3( a )  is, in the cylindrical nonaqueous electrolyte secondary battery according to the example of the embodiment, a front view showing a state in which a sealing body is welded to a positive electrode lead, and  FIG. 3( b )  is a side view of  FIG. 3( a ) . 
         FIG. 4( a )  is a plan view of an upper insulating plate according to the example of the embodiment, and  FIG. 4( b )  is a front view of the upper insulating plate according to the example of the embodiment. 
         FIG. 5( a )  is a plan view of an upper insulating plate according to a comparative example, and  FIG. 5( b )  is a front view of the upper insulating plate according to the comparative example. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, an embodiment according to the present invention will be described in detail with reference to the attached drawings. In the following description, concrete shapes, materials, values, numbers, directions, and the like will be described by way of example in order to facilitate the understanding of the present invention and may be appropriately changed or modified in accordance with a specification of a nonaqueous electrolyte secondary battery. In addition, the term “approximately” to be described below is used, for example, to indicate, besides the case of exactly the same, the case of substantially the same. 
       FIG. 1  is a schematic cross-sectional view of a cylindrical nonaqueous electrolyte secondary battery  10  which is one example of the embodiment.  FIG. 2  is an enlarged view of an A portion of  FIG. 1 . As shown in  FIGS. 1 and 2 , the cylindrical nonaqueous electrolyte secondary battery  10  includes a winding type electrode group  14  and a nonaqueous electrolyte (not shown). The winding type electrode group  14  includes a positive electrode (not shown), a negative electrode  12 , and at least one separator (not shown), and the positive electrode and the negative electrode  12  are spirally wound with the separator interposed therebetween. Hereinafter, one side in an axial direction of the electrode group  14  and the other side in the axial direction thereof are called “upper” and “lower”, respectively, in some cases. The nonaqueous electrolyte contains a nonaqueous solvent and an electrolyte salt dissolved therein. The nonaqueous electrolyte is not limited to a liquid electrolyte and may be a solid electrolyte using a gel polymer or the like. Hereinafter, the cylindrical nonaqueous electrolyte secondary battery  10  will be described as the secondary battery  10 . 
     The positive electrode includes a belt-shaped positive electrode collector (not shown). To the positive electrode collector, one end (lower end shown in  FIG. 1 ) of a positive electrode lead  16  is bonded. The positive electrode lead  16  is an electrically conductive member to electrically connect the positive electrode collector to a positive electrode terminal and extends from an upper end of the electrode group  14  to one side (upper side) of the electrode group  14  in an axial direction a. The one end of the positive electrode lead  16  is bonded to a portion of the positive electrode collector located, for example, at an approximately central portion of the electrode group  14  in a radius direction P. In addition, the other end (upper end shown in  FIG. 1 ) of the positive electrode lead  16  is bonded to an approximately center of a lower surface of a sealing body  22 . 
     The negative electrode  12  includes a belt-shaped negative electrode collector  13 . To the negative electrode collector  13 , a negative electrode lead (not shown) is bonded. The negative electrode lead is an electrically conductive member to electrically connect the negative electrode collector  13  to a negative electrode terminal and extends from a lower end of the electrode group  14  to the other side (lower side) thereof in the axial direction a. For example, the negative electrode lead is provided at a winding start-side end portion of the electrode group  14 . A lower end of the negative electrode lead is bonded to a bottom portion of a bottom-closed cylindrical exterior package can  20 . In  FIG. 1 , the negative electrode  12  is exposed as the outermost circumferential surface of the electrode group  14 , and the outermost circumferential surface of this negative electrode  12  is in contact with an inner circumferential surface of the exterior package can  20 . Accordingly, the negative electrode  12  of the secondary battery  10  is connected to the exterior package can  20  which functions as the negative electrode terminal. 
     The positive electrode lead  16  and the negative electrode lead are each a belt-shaped electrically conductive member having a thickness larger than that of the collector. The thickness of each lead is, for example, 3 to 30 times the thickness of the collector and is generally 50 to 500 μm. A material forming each lead is not particularly limited. The positive electrode lead  16  is preferably formed of a metal containing aluminum as a primary component. The negative electrode lead is preferably formed of a metal containing nickel or copper as a primary component or a metal containing both nickel and copper. Alternatively, the negative electrode  12  is not exposed as the outermost circumferential surface of the electrode group  14 , and a negative electrode lead is bonded to a winding finish-side end portion of the negative electrode collector and is allowed to extend from the lower end of the electrode group  14  to the other side thereof in the axial direction a, so that the two negative electrode leads may be bonded to the bottom portion of the exterior package can  20 . 
     The positive electrode and the negative electrode  12  will be described in more detail. The positive electrode includes the belt-shaped positive electrode collector and at least one positive electrode active material layer formed thereon. For example, on each of two surfaces of the positive electrode collector, the positive electrode active material layer is formed. As the positive electrode collector, foil formed of a metal, such as aluminum, or a film having a surface layer formed of the metal mentioned above may be used. As a preferable positive electrode collector, metal foil containing aluminum or an aluminum alloy as a primary component may be mentioned. The thickness of the positive electrode collector is, for example, 10 to 30 μm. 
     The positive electrode active material layer is preferably formed on the entire region of each of the two surfaces of the positive electrode collector other than a bare portion to which the positive electrode lead is to be bonded. The positive electrode active material layer preferably contains a positive electrode active material, an electrically conductive agent, and a binder. The positive electrode is formed such that a positive electrode mixture slurry containing the positive electrode active material, the electrically conductive agent, the binder, and a solvent, such as N-methyl-2-pyrrolidone (NMP), is applied on the two surfaces of the positive electrode collector, followed by drying and rolling. 
     As the positive electrode active material, for example, a lithium transition metal oxide containing at least one transition metal element selected, for example, from Co, Mn, and Ni may be mentioned. Although the lithium transition metal oxide is not particularly limited, a composite oxide represented by a general formula of Li 1+x MO 2  (in the formula, −0.2&lt;x≤0.2 is satisfied, and M represents at least one of Ni, Co, Mn, and Al) is preferable. 
     As an example of the electrically conductive agent, for example, a carbon material, such as carbon black (CB), acetylene black (AB), Ketjen black, or graphite, may be mentioned. As an example of the binder, for example, there may be mentioned a fluorinated resin, such as a polytetrafluoroethylene (PTFE) or a poly(vinylidene fluoride) (PVdF), a polyacrylonitrile (PAN), a polyimide (PI), an acrylic resin, or a polyolefinic resin. In addition, together with at least one of those resins mentioned above, a carboxymethyl cellulose (CMC) or its salt, a poly(ethylene oxide) (PEO), or the like may be used in combination. Those resins mentioned above may be used alone, or at least two types thereof may be used in combination. 
     The negative electrode  12  includes the belt-shaped negative electrode collector  13  and at least one negative electrode active material layer formed thereon. For example, on each of two surfaces of the negative electrode collector  13 , the negative electrode active material layer is formed. As the negative electrode collector  13 , foil formed of a metal, such as copper, or a film having a surface layer formed of the metal mentioned above may be used. The thickness of the negative electrode collector  13  is, for example, 5 to 30 μm. 
     The negative electrode active material layer is preferably formed on the entire region of each of the two surfaces of the negative electrode collector  13  other than a bare portion to which the negative electrode lead is to be bonded. The negative electrode active material layer preferably contains a negative electrode active material and a binder. The negative electrode  12  is formed such that a negative electrode mixture slurry containing the negative electrode active material, the binder, and water or the like is applied on the two surfaces of the negative electrode collector  13 , followed by drying and rolling. 
     As the negative electrode active material, any material may be used as long as capable of reversibly occluding and releasing lithium ions, and for example, there may be used a carbon material, such as natural graphite or artificial graphite, a metal, such as Si or Sn, forming an alloy with lithium, or an alloy or a composite oxide containing at least one of those mentioned above. As the binder contained in the negative electrode active material layer, for example, a resin similar to that in the case of the positive electrode  11  may be used. When the negative electrode mixture slurry is prepared using an aqueous solvent, for example, a styrene-butadiene rubber (SBR), a CMC or its salt, a polyacrylic acid or its salt, or a poly(vinyl alcohol) may be used. Those resins mentioned above may be used alone, or at least two types thereof may be used in combination. 
     In the example shown in  FIG. 1 , by the exterior package can  20  and the sealing body  22 , a metal-made battery case receiving the electrode group  14  and the nonaqueous electrolyte is formed. Between the exterior package can  20  and the sealing body  22 , a gasket  24  is provided, and hence air tightness in the battery case is secured. The exterior package can  20  has a groove portion  21  formed, for example, by pressing a side surface portion from the outside to support the sealing body  22 . The groove portion  21  is preferably formed to have a ring shape along a circumferential direction of the exterior package can  20  and supports the sealing body  22  by an upper surface thereof. 
     In  FIG. 1 , the sealing body  22  is schematically shown as a round shape having a rectangular cross-section. For example, the sealing body  22  is composed of a filter, a lower valve, an insulating member, an upper valve, and a cap, which are laminated in this order from an electrode group  14  side. The members forming the sealing body  22  each have, for example, a disc shape or a ring shape, and the members except for the insulating member are electrically connected to each other. The lower valve and the upper valve are connected to each other at central portions thereof, and the insulating member is provided between peripheral portions thereof. When the inside pressure of the battery is increased by abnormal heat generation, for example, the lower valve is fractured, and as a result, the upper valve is expanded to a cap side and is separated from the lower valve, so that the electrical connection between the two valves is blocked. When the inside pressure is further increased, the upper valve is fractured, and gas is discharged through an opening portion formed in the cap. 
     At an upper side of the electrode group  14 , an upper insulating plate  26  is disposed. In  FIG. 1 , although the upper insulating plate  26  is shown to be apart from the electrode group  14 , the upper insulating plate  26  is actually disposed to be in contact with the upper end of the electrode group  14 . The positive electrode lead  16  is allowed to extend to a sealing body  22  side through a lead hole  27  which is a through-hole of the upper insulating plate  26  and is welded to a lower surface of the sealing body  22 . In the secondary battery  10 , a top plate of the sealing body  22  or the cap located at an upper end is used as the positive electrode terminal. 
       FIG. 3( a )  is a front view showing, in the secondary battery  10 , a state in which the sealing body  22  is welded to the positive electrode lead  16 , and  FIG. 3( b )  is a side view of  FIG. 3( a ) . In  FIG. 3 , as is the case shown in  FIG. 1 , the sealing body  22  is also schematically shown to have a disc shape. As shown in  FIG. 3 , when the positive electrode lead  16  is welded to the sealing body  22 , the sealing body is disposed to be overlapped with the positive electrode lead  16  extending from the electrode group  14 . In addition, by laser welding or the like, the positive electrode lead  16  is welded to the sealing body  22 . In the positive electrode lead  16 , as shown in  FIG. 2 , an insulating tape  17  is adhered to a portion surrounded by a dotted line in  FIG. 1 . 
     After the positive electrode lead  16  is welded to the sealing body  22  as described above, the sealing body  22  is fitted to an upper portion of the exterior package can  20 . In this step, the positive electrode lead  16  is bent at a position adjacent to the lead hole  27  to form a first curved section  16   a . Furthermore, the positive electrode lead  16  is folded back at a position opposite to the first curved section  16   a  with respect to a central axis O of the secondary battery  10  orthogonal to the sealing body  22  to form a second curved section  16   b . As shown in  FIGS. 1 and 2 , the insulating tape  17  is adhered to the positive electrode lead  16 . In order not to disturb the welding between the sealing body  22  and the positive electrode lead  16 , the insulating tape  17  is preferably adhered to a region of the positive electrode lead  16  from the electrode group  14  side toward the sealing body  22  side so as not to extend past an inflection point of the second curved section  16   b . In addition, the insulating tape may be adhered not only to a portion of the positive electrode lead  16  extending from the electrode group  14  but also to a portion of the positive electrode lead  16  disposed in the electrode group  14  or may be adhered only to a surface of the positive electrode lead  16  facing the upper insulating plate  26 . In addition, the insulating tape may be adhered so as to be spirally wound around the portion of the positive electrode lead  16  surrounded by the dotted line shown in  FIG. 1 . 
     The secondary battery  10  may be compressively deformed, for example, by a crushing test in some cases. In this case, when an opening portion  28  is formed in the upper insulating plate  26  at a second curved section  16   b  side as described later, the second curved section  16   b  comes in contact with the electrode group  14  through the opening portion  28 , so that short circuit may probably occur in some cases. In this embodiment, in order to effectively prevent this short circuit, as described later, the position of the opening portion  28  of the upper insulating plate  26  is appropriately restricted. 
     In addition, in the exterior package can  20 , between the lower end of the electrode group  14  and the bottom portion of the exterior package can  20 , a lower insulating plate (not shown) is disposed. In a center portion of the lower insulating plate, a through-hole is formed. The negative electrode lead (not shown) bonded to the one end of the negative electrode collector  13  is allowed to extend to a lower side of the lower insulating plate through the through-hole thereof or along an outer circumferential side of the lower insulating plate and is then bonded to the bottom portion of the exterior package can  20  by welding. 
     The upper insulating plate  26  will be described in detail with reference to  FIG. 4 .  FIG. 4( a )  is a plan view of the upper insulating plate  26 , and  FIG. 4( b )  is a front view thereof. The upper insulating plate  26  has a round shape with a small thickness t. The upper insulating plate  26  is formed, for example, from an insulating material, such as a glass cloth phenol containing a phenol resin and a glass fiber base material impregnated therewith. In one half portion (lower half portion in  FIG. 4( a ) ) of the upper insulating plate  26 , an arc-shaped lead hole  27  having an approximately half circle is formed. On the other hand, in the other half portion (upper half portion in  FIG. 4( a ) ) of the upper insulating plate  26 , along an intermediate portion in a radius direction, the opening portions  28  each having an oval shape are formed at positions apart from each other in a circumferential direction. A maximum length La of the opening portion  28 , which is the width thereof in the circumferential direction, along the longitudinal direction of the opening portion  28  is preferably set to be smaller than a width Lb ( FIG. 3( a ) ) of the positive electrode lead  16  (La&lt;Lb). 
     The distances from the center of the upper insulating plate  26  to the opening portions  28  are the same. In addition, in the center of the upper insulating plate  26 , a central hole  29  having an approximately oval shape is formed. The opening portions  28 , the central hole  29 , and the lead hole  27  are each preferably formed larger in order to improve the emission performance when gas is generated in the secondary battery  10 . 
     In addition, as shown in  FIG. 1 , in the state in which the upper insulating plate  26  is disposed in the secondary battery  10 , in the upper insulating plate  26 , the four opening portions  28  are formed at positions opposite to the lead hole  27  with respect to the central axis O of the secondary battery. When the positive electrode lead  16  and the upper insulating plate  26  are viewed from the sealing body  22  side (upper side in  FIG. 1 ), since the central hole  29  is formed so as to be overlapped with a hollow portion of the electrode group  14 , the probability of short circuit caused by contact between the positive electrode lead  16  and the electrode group  14  through the central hole  29  is low. However, when the positive electrode lead  16  and the upper insulating plate  26  are viewed from the sealing body  22  side, the central hole  29  is preferably formed at a position so as to be overlapped with the portion at which the insulating tape  17   b  is adhered to the positive electrode lead  16 . Furthermore, when the distance from the central axis O of the secondary battery  10  to the second curved section  16   b  (distance from the central axis O to a portion of the second curved section  16   b  farthest therefrom) is represented by L 1 , and when the distance from the central axis O of the secondary battery  10  to the opening portion  28  (distance from the central axis O to a part of the opening portion  28  nearest thereto) is represented by L 2 , L 1  and L 2  are restricted so as to satisfy L 2 &gt;L 1 . As long as the restriction described above is satisfied, the position and the shape of the opening portion  28  are not limited to those of this embodiment. 
     Furthermore, in the upper insulating plate  26 , although opening ratios of all the openings including the opening portion  28 , the central hole  29 , and the lead hole  27  are not particularly limited, the ratios are each preferably 20% or more. Although the upper limit of the opening ratio may be appropriately determined in accordance with the strength of the upper insulating plate  26 , for example, the upper limit may be set to 60% or less. 
     According to the secondary battery  10  described above, when the distance from the central axis O of the secondary battery  10  to the second curved section  16   b  of the positive electrode lead  16  is set to be L 1 , and when the distance from the central axis O of the secondary battery  10  to the opening portion  28  is set to be L 2 , the distances L 1  and L 2  are restricted so as to satisfy L 2 &gt;L 1 . Hence, while the discharge performance of internal gas is secured, the short circuit between the electrode group  14  and the positive electrode lead  16  can be effectively prevented. 
     In addition, in the upper insulating plate  26 , when the maximum length La ( FIG. 4 ) of each opening portion  28  is set to be smaller than the width Lb ( FIG. 3( a ) ) of the positive electrode lead  16 , even if the curved section of the positive electrode lead  16  is deformed to the electrode group side by a crushing test, the short circuit between the electrode group and the positive electrode lead  16  can be sufficiently suppressed. 
     Furthermore, the opening ratio of the upper insulating plate  26  is 20% or more. Accordingly, the emission performance of internal gas can be further improved. 
     Experimental Example 
     The inventor of the present disclosure formed secondary batteries of an example and a comparative example as described below and then performed a crushing test. 
     Example 
     [Formation of Positive Electrode] 
     As a positive electrode active material, an aluminum-containing lithium nickel cobalt oxide represented by LiNi 0.88 Co 0.09 Al 0.03 O 2  was used. Subsequently, 100 parts by weight of LiNi 0.88 Co 0.09 Al 0.03 O 2 , 1.0 part by weight of acetylene black, and 0.9 parts by weight of a poly(vinylidene fluoride) (PVdF) (binder) were mixed in a solvent of N-methyl-2-pyrrolidone (NMP), so that a positive electrode mixture slurry was obtained. This positive electrode mixture slurry in the form of paste was uniformly applied on two surfaces of a long positive electrode collector formed from aluminum foil having a thickness of 15 μm. Next, in a heated dryer, after the positive electrode collector on which the coating films were formed was heat-treated at a temperature of 100° C. to 150° C. to remove NMP, rolling was performed using a roll press machine to form a positive electrode active material layer, and furthermore, after the rolling was performed, a positive electrode was brought into contact with at least one roller heated to 200° C. for 5 seconds to perform a heat treatment. In addition, the positive electrode collector on which positive electrode active material layers were formed was cut into a predetermined electrode size to form the positive electrode, and next, an aluminum-made positive electrode lead  16  was fitted on the positive electrode collector. The thickness, the width, and the length of the positive electrode thus formed were 0.144 mm, 62.6 mm, and 861 mm, respectively. In addition, the width, the thickness, and the length of the positive electrode lead  16  were 3.5 mm, 0.15 mm, and 76 mm, respectively. 
     [Formation of Negative Electrode] 
     As a negative electrode active material, there was used a mixture obtained by mixing 94 parts by weight of a graphite powder and 6 parts by weight of mother particles containing a lithium silicate phase represented by Li 2 Si 2 O 5  and silicon particles dispersed therein. Subsequently, the mixture described above, 1 part by weight of a carboxymethyl cellulose (CMC) as a thickening agent, and 1 part by weight of a dispersion of a styrene-butadiene rubber as a binder were dispersed in water, so that a negative electrode mixture slurry was prepared. This negative electrode mixture slurry was applied on two surfaces of a negative electrode collector formed from copper foil having a thickness of 8 m to form negative electrode coating portions. Next, after coating films were dried in a heated dryer, the thickness of a negative electrode active material layer was adjusted by compression using compression rollers so that the thickness of a negative electrode was 0.160 mm. In addition, the negative electrode collector on which the negative electrode active material layers were formed was cut into a predetermined electrode size to form a negative electrode  12 , and subsequently, a nickel-copper-nickel-made negative electrode lead was fitted on the negative electrode collector. The negative electrode thus formed had a width of 64.2 mm and a length of 959 mm. 
     [Formation of Battery Electrode Group] 
     An electrode group  14  was formed by spirally winding the positive electrode and the negative electrode  12  with polyethylene-made separators interposed therebetween. 
     [Preparation of Nonaqueous Electrolyte Liquid] 
     After 4 parts by weight of vinylene carbonate (VC) was added to 100 parts by weight of a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), and dimethyl methyl carbonate (DMC) (volume ratio: EC:FEC:DMC=1:1:3), LiPF 6  was dissolved in this mixed solvent to obtain a concentration of 1.5 mole/L, so that a nonaqueous electrolyte liquid was prepared. To 100 parts by weight of the nonaqueous electrolyte liquid thus prepared, a predetermined amount of a boric acid ester compound was added to form a nonaqueous electrolyte liquid for a secondary battery. 
     [Formation of Upper Insulating Plate] 
     As an upper insulating plate  26 , a round-shaped plate member formed from a glass cloth phenol having a thickness t of 0.3 mm was used, a lead hole  27  through which a positive electrode lead  16  was to penetrate, a central hole  29 , and four opening portions  28  were formed. The four opening portions  28  were formed at four positions located at a side opposite to the lead hole  27  with respect to the center of the upper insulating plate  26  and were separated from each other in a circumferential direction of the upper insulating plate  26 . 
     [Formation of Secondary Battery] 
     After the upper insulating plate  26  and a lower insulating plate were disposed at an upper side and a lower side of the electrode group  14 , respectively, the electrode group  14  was received in an exterior package can  20 . The positive electrode lead  16  extended from the electrode group  14  through the lead hole  27  of the upper insulating plate  26 . The negative electrode lead was welded to the exterior package can  20  of a battery case, and the positive electrode lead  16  was welded to a sealing body including an inner pressure sensitive safety valve. Subsequently, the nonaqueous electrolyte liquid was charged in the battery case by a reduced pressure method. Finally, a sealing body  22  was caulked at an upper opening end portion of the exterior package can  20  with a gasket  24  interposed therebetween, so that a secondary battery  10  was formed. The capacity of the secondary battery  10  was 4,600 mAH. As shown in  FIG. 1 , the center of the upper insulating plate  26  was located at the central axis O of the secondary battery  10 , and after the first curved section  16   a  and the second curved section  16   b  of the positive electrode lead  16  were formed, the positive electrode lead  16  was received in the battery case. As described above, in the state in which the positive electrode lead  16  was received in the battery case, the distance L 1  from the central axis O of the secondary battery  10  to the second curved section  16   b  was 5.3 mm, and the distance L 2  from the central axis O of the secondary battery  10  to the opening portion  28  was 5.9 mm. 
     Comparative Example 
       FIG. 5( a )  is a plan view of an upper insulating plate  26   a  of a comparative example, and  FIG. 5( b )  is a front view of the upper insulating plate  26   a  of the comparative example. As shown in  FIG. 5 , by using a round-shaped plate member formed from a glass cloth phenol having a thickness t of 0.3 mm, a lead hole  27   a  through which a positive electrode lead was to penetrate, a central hole  29 , and three opening portions  28   a  were formed, so that the upper insulating plate  26   a  according to the comparative example was formed. The three opening portions  28   a  were formed at three positions located at a side opposite to the lead hole  27   a  with respect to the center of the upper insulating plate and were separated from each other in a circumferential direction of the upper insulating plate  26   a . Except for that the upper insulating plate  26   a  was used, the distance L 1  from the central axis of the secondary battery to the second curved section  16   b  of the positive electrode lead  16  was set to 5.3 mm, and the distance L 2  from the central axis of the secondary battery to the opening portion  28   a  was set to 5.2 mm, a secondary battery according to the comparative example was formed in a manner similar to that of the example. 
     [Crushing Test] 
     By using the example and the comparative example, the influence of the distance L 1  from the central axis of the secondary battery to the second curved section and the distance L 2  from the central axis of the secondary battery to each of the opening portions  28  and  28   a  on the generation of short circuit caused by contact between the positive electrode lead  16  and the electrode group was investigated. For this investigation, a crushing test was performed in accordance with the following procedure from (1) to (3). 
     (1) In each of the example and the comparative example, a partially charged secondary battery was used.
 
(2) The secondary battery was disposed between two flat plates, and a load was to be applied from the side of the secondary battery by a crushing device. After reaching a target applying force, the load thus applied was maintained for one minute and was then released. The crushing test was performed at target applying forces of 13 kN and 20 kN.
 
(3) In this test, when the temperature of the secondary battery was increased to 40° C. or more, it was judged that heat generation occurred by the short circuit between the positive electrode lead  16  and the electrode group  14 . The test results are shown in Table 1.
 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 TARGET APPLYING 
                 TARGET APPLYING 
               
               
                   
                 FORCE 
                 FORCE 
               
               
                   
                 13 kN 
                 20 kN 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 COMPARATIVE 
                 0/5  
                 1/5  
               
               
                 EXAMPLE 
               
               
                 EXAMPLE 
                 0/20 
                 0/20 
               
               
                   
               
            
           
         
       
     
     In Table 1, in the comparative example and the example, the rates of heat generation by the contact between the second curved section  16   b  of the positive electrode lead  16  and the electrode group  14  are shown at target applying forces of 13 kN and 20 kN. For example, in Table 1, “0/5” indicates that among five crushing tests, the number of test results indicating the heat generation was zero. 
     In the example, in the tests performed at the two target applying forces, the heat generation caused by the short circuit between the second curved section  16   b  of the positive electrode lead  16  and the negative electrode of the electrode group  14  through the opening portion  28  of the upper insulating plate  26  was not observed. Accordingly, as shown in Table 1, in the example, at the both target applying forces, no heat generation was observed among 20 crushing tests. 
     On the other hand, in the comparative example, by the test at a target applying force of 13 kN, no heat generation was observed. However, in the test at a target applying force of 20 kN, by the short circuit between the second curved section  16   b  of the positive electrode lead  16  and the negative electrode of the electrode group  14 , at a fifth test, the temperature of the secondary battery was increased close to 120° C., and hence, the heat generation was observed. In the comparative example, since the heat generation was observed at the fifth test, a sixth test or more was not performed. 
     From the test results described above, since the opening portions  28  were formed in the upper insulating plate  26  at an outer circumferential side than the second curved section  16   b  of the positive electrode lead  16  as shown in the example, an effect of preventing the short circuit caused by the positive electrode lead  16  which intrudes into the electrode group  14  through the opening portion  28  could be confirmed. 
     In addition, although the case in which the central hole  29  is formed at the center of the upper insulating plate  26  has thus been described, the structure of the present disclosure may also be applied to the structure having no central hole. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10  cylindrical nonaqueous electrolyte secondary battery (secondary battery),  12  negative electrode,  14  electrode group,  16  positive electrode lead,  16   a  first curved section,  16   b  second curved section,  17  insulating tape,  20  exterior package can,  21  groove portion,  22  sealing body,  24  gasket,  26 ,  26   a  upper insulating plate,  27  lead hole,  28 ,  28   a  opening portion,  29  central hole