Patent Publication Number: US-2020303782-A1

Title: Nonaqueous electrolyte secondary battery

Description:
TECHNICAL FIELD 
     The present disclosure relates to a nonaqueous electrolyte secondary battery. 
     BACKGROUND ART 
     Patent Document 1 has disclosed an insulating tape to be used for a nonaqueous electrolyte secondary battery, the insulating tape including an adhesive layer and an inorganic particle-containing layer which contains inorganic particles. In addition, Patent Document 1 has also disclosed a usage mode in which the insulating tape described above is adhered to a lead which is used for electrical connection between a terminal and a collector of an electrode. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: Japanese Published Unexamined Patent Application No. 2006-093147 
     SUMMARY OF INVENTION 
     Technical Problem 
     Incidentally, since a current is concentrated to a positive electrode lead bonded to a positive electrode collector, heat is liable to be generated particularly at a portion (hereinafter, referred to as “extension portion” in some cases) of the positive electrode lead extending past one end of the collector. Since the extension portion of the positive electrode lead partially faces a negative electrode with a separator interposed therebetween, when heat generation is increased at the extension portion by a large current flowing through the positive electrode lead due to an external short circuit or the like, an internal short circuit may be generated by melting of the separator. In addition, an electrically conductive foreign material intruding between the extension portion of the positive electrode lead and the negative electrode may break through the separator, and an internal short circuit may be generated in some cases. 
     Solution to Problem 
     A nonaqueous electrolyte secondary battery according to one aspect of the present disclosure comprises: a wound electrode assembly in which a positive electrode and a negative electrode are wound with at least one separator interposed therebetween; the positive electrode includes a belt-shaped positive electrode collector and a positive electrode lead bonded to the positive electrode collector; an insulating tape is adhered to, of a portion of the positive electrode lead extending past one end of the positive electrode collector, at least a range facing the negative electrode with the separator interposed therebetween; the insulating tape includes a base material layer, an adhesive layer, and an inorganic particle-containing layer formed therebetween; and the inorganic particle-containing layer contains 20 percent by weight or more of inorganic particles with respect to the weight of the layer described above. 
     Advantageous Effects of Invention 
     According to the nonaqueous electrolyte secondary battery of the present disclosure, an internal short circuit to be generated by melting of the separator caused by heat generation of the extension portion of the positive electrode lead can be highly suppressed. In addition, an internal short circuit to be generated by an electrically conductive foreign material which intrudes between the negative electrode and the extension portion of the positive electrode lead can also be highly suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of a nonaqueous electrolyte secondary battery according to one example of an embodiment. 
         FIG. 2  is a perspective view of a wound electrode assembly according to one example of the embodiment. 
         FIG. 3  is a front view of a positive electrode and a negative electrode which collectively form the electrode assembly. 
         FIG. 4  is a cross-sectional view of the vicinity of a positive electrode lead of the electrode assembly. 
         FIG. 5  is a cross-sectional view of an insulating tape according to one example of the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As described above, when the heat generation of the extension portion of the positive electrode lead is increased due to an external short circuit or the like, the separator is melted, and an internal short circuit may be generated in some cases. In association with an increase in capacity and an increase in output of the battery, since an increase in length of the positive electrode, a decrease in thickness of the separator, an increase in thickness and width of the positive electrode lead, and the like are performed, it becomes more important to overcome the internal short circuit as described above. As measures to overcome the internal short circuit as described above, for example, adhesion of the insulating tape disclosed in Patent Document 1 to the extension portion of the positive electrode lead has been considered. According to a tape including an inorganic particle-containing layer and an adhesive layer, such as the tape disclosed in Patent Document 1, there is a trade-off relationship in which although a heat resistance can be improved by increasing the addition amount of inorganic particles, a piercing strength is decreased when the addition amount thereof is increased, and as a result, the internal short circuit caused by an electrically conductive foreign material cannot be sufficiently suppressed. 
     Through intensive research carried out by the present inventors to prevent the individual internal short circuits described above, a new electrode assembly which uses an insulating tape formed of at least three layers, that is, a base material layer/an inorganic particle-containing layer containing 20 percent by weight or more of inorganic particles/an adhesive layer was found. The insulating tape having a three-layer structure as described above is excellent in heat resistance and also has a high piercing strength. When the insulating tape as described above is adhered to, of the extension portion of the positive electrode lead, a range facing the negative electrode with the separator interposed therebetween, the above individual internal short circuits can be highly suppressed, and the heat generation of the battery caused by duration of the short circuits can also be suppressed. 
     Hereinafter, one example of an embodiment will be described in detail. 
     The drawings to be used for illustrating the embodiment are schematically drawn, and hence, particular dimensional ratios and the like are to be understood in consideration of the following description. In this specification, when the term “approximately” is explained using approximately the same by way of example, the “approximately the same” is intentionally used to include not only “completely the same” but also “substantially the same”. In addition, the term “end portion” indicates the end of an object and the vicinity thereof, and the term “central portion” indicates the center of an object and the vicinity thereof. 
     Although a nonaqueous electrolyte secondary battery  10  which is a cylindrical battery including a cylindrical metal-made case will be described as one example of the embodiment, a nonaqueous electrolyte secondary battery of the present disclosure is not limited thereto. The nonaqueous electrolyte secondary battery of the present disclosure may be, for example, either a prismatic battery including a prismatic metal-made case or a laminate battery including an exterior package body formed of resin-made sheets. 
       FIG. 1  is a cross-sectional view of the nonaqueous electrolyte secondary battery  10 .  FIG. 2  is a perspective view of an electrode assembly  14  forming the nonaqueous electrolyte secondary battery  10 . As illustrated in  FIGS. 1 and 2 , the nonaqueous electrolyte secondary battery  10  includes the wound electrode assembly  14  and a nonaqueous electrolyte (not shown). The wound electrode assembly  14  includes a positive electrode  11 , a negative electrode  12 , and at least one separator  13 , and the positive electrode  11  and the negative electrode  12  are spirally wound with the separator  13  interposed therebetween. Hereinafter, one axial direction of the electrode assembly  14  is called “upper side”, and the other axial direction is called “lower side” in some cases. The nonaqueous electrolyte includes a nonaqueous solvent and an electrolyte salt dissolved in the nonaqueous solvent. The nonaqueous electrolyte is not limited to a liquid electrolyte and may also be a solid electrolyte using a gel polymer or the like. 
     The positive electrode  11  includes a belt-shaped positive electrode collector  30  (see  FIG. 3  which will be described below) and a positive electrode lead  19  bonded to the above collector. The positive electrode lead  19  is an electrically conductive member electrically connecting the positive electrode collector  30  and a positive electrode terminal and extends past an upper end of an electrode group in an axial direction α (upper side) of the electrode assembly  14 . In this case, the electrode group indicates the electrode assembly  14  other than the leads. The positive electrode lead  19  is provided at an approximately central portion of the electrode assembly  14  in a radial direction β thereof. 
     The negative electrode  12  includes a belt-shaped negative electrode collector  35  (see  FIG. 3  which will be described below) and negative electrode leads  20   a  and  20   b  connected to the above collector. The negative electrode leads  20   a  and  20   b  are electrically conductive members electrically connecting the negative electrode collector  35  and a negative electrode terminal and extend past a lower end of the electrode group in the axial a (lower side). For example, the negative electrode lead  20   a  is provided at a winding-start side end portion of the electrode assembly  14 , and the negative electrode lead  20   b  is provided at a winding-finish side end portion of the electrode assembly  14 . 
     The positive electrode lead  19  and the negative electrode leads  20   a  and  20   b  are each a belt-shaped electrically conductive member having a thickness larger than that of the collector. The thickness of the lead is, for example, 3 to 30 times the thickness of the collector and is generally 50 to 500 nm. Although a constituent material of each lead is not particularly limited, the positive electrode lead  19  is preferably formed from a metal containing aluminum as a primary component, and the negative electrode leads  20   a  and  20   b  are each preferably formed from a metal containing nickel or copper as a primary component. In addition, the number of the leads, the arrangement thereof, and the like are not particularly limited. For example, at least two positive electrode leads  19  may be provided. 
     In the example shown in  FIG. 1 , a metal-made battery case receiving the electrode assembly  14  and the nonaqueous electrolyte is formed by a case main body  15  and a sealing body  16 . Insulating plates  17  and  18  are provided at an upper side and a lower side of the electrode assembly  14 , respectively. The positive electrode lead  19  extends to a sealing body  16  side through a through-hole of the insulating plate  17  and is welded to a bottom surface of a filter  22  which is a bottom plate of the sealing body  16 . In the nonaqueous electrolyte secondary battery  10 , a cap  26  which is a top plate of the sealing body  16  electrically connected to the filter  22  is used as the positive electrode terminal. On the other hand, the negative electrode lead  20   a  which passes through a through-hole of the insulating plate  18  and the negative electrode lead  20   b  which passes along the outside of the insulating plate  18  each extend to a bottom portion side of the case main body  15  and are then welded to an inner surface of the bottom portion of the case main body  15 . In the nonaqueous electrolyte secondary battery  10 , the case main body  15  functions as the negative electrode terminal. 
     As described above, the electrode assembly  14  has a winding structure in which the positive electrode  11  and the negative electrode  12  are spirally wound with the separator  13  interposed therebetween. The positive electrode  11 , the negative electrode  12 , and the separator  13  are each formed to have a belt shape and are spirally wound so as to be alternately laminated to each other in the radial direction β of the electrode assembly  14 . In the electrode assembly  14 , the longitudinal direction of each electrode is a winding direction γ, and the width direction of each electrode is the axial direction α. In this embodiment, a space  28  is formed in a winding core of the electrode assembly  14 . 
     Although the details will be described later, the electrode assembly  14  includes at least one insulating tape  40  adhered to the positive electrode lead  19 . The insulating tape  40  is adhered to, of an extension portion P 1  which is a portion of the positive electrode lead  19  extending past one end of the positive electrode collector  30 , at least a range (hereinafter, referred to as “facing region” in some cases) facing the negative electrode  12  with the separator  13  interposed therebetween. In this embodiment, the insulating tape  40  is adhered to a range wider than the facing region of the positive electrode lead  19 . 
     The case main body  15  is a cylindrical metal-made container having a bottom plate. A gasket  27  is provided between the case main body  15  and the sealing body  16  so that air tightness in the battery case is secured. The case main body  15  has a protruding portion  21  which is formed, for example, by pressing a side surface portion from the outside and which supports the sealing body  16 . The protruding portion  21  is preferably formed to have an annular shape along a circumference direction of the case main body  15 , and an upper surface of the protruding portion  21  supports the sealing body  16 . 
     The sealing body  16  includes the filter  22 , a lower valve body  23 , an insulating member  24 , an upper valve body  25 , and the cap  26  laminated in this order from an electrode assembly  14  side. The individual members forming the sealing body  16  each have, for example, a circular plate shape or a ring shape, and the members other than the insulating member  24  are electrically connected to each other. The lower valve body  23  and the upper valve body  25  are connected to each other at the central portions thereof, and between the peripheral portions of the valve bodies, the insulating member  24  is provided. When the inside pressure of the battery is increased by abnormal heat generation, for example, since the lower valve body  23  is fractured, the upper valve body  25  is expanded to a cap  26  side and is separated from the lower valve body  23 , so that the electrical connection between the valve bodies is disconnected. When the inside pressure is further increased, the upper valve body  25  is fractured, and a gas is exhausted from an opening portion of the cap  26 . 
     Hereinafter, with reference to  FIGS. 3 and 4 , the electrode assembly  14 , in particular, the insulating tape  40  to be adhered to the positive electrode  11  and the positive electrode lead  19 , will be described in detail.  FIG. 3  is a front view of the positive electrode  11  and the negative electrode  12  which collectively form the electrode assembly  14 . In  FIG. 3 , the state in which the electrodes are each linearly extended is shown, and the right side on the plane is the winding-start side of the electrode assembly  14 , and the left side on the plane is the winding-finish side of the electrode assembly  14 .  FIG. 4  is a cross-sectional view of the vicinity of the winding core of the electrode assembly  14 . 
     As illustrated in  FIGS. 3 and 4 , in the electrode assembly  14 , in order to prevent precipitation of lithium on the negative electrode  12 , the negative electrode  12  is formed to have a size larger than that of the positive electrode  11 . In addition, a portion at which a positive electrode active material layer  31  of the positive electrode  11  is formed is at least disposed to face a portion at which a negative electrode active material layer  36  of the negative electrode  12  is formed with the separator  13  interposed therebetween. The width and the length of the negative electrode collector  35  determining the size of the negative electrode  12  are set so as to be larger than the width and the length of the positive electrode collector  30  determining the size of the positive electrode  11 . 
     The positive electrode  11  includes the belt-shaped positive electrode collector  30  and at least one positive electrode active material layer  31  formed on the above collector. In this embodiment, the positive electrode active material layers  31  are formed on two surfaces of the positive electrode collector  30 . As the positive electrode collector  30 , foil of a metal, such as aluminum, a film having a surface layer on which the metal mentioned above is disposed, or the like is used. A preferable positive electrode collector  30  is foil of a metal containing aluminum or an aluminum alloy as a primary component. The thickness of the positive electrode collector  30  is, for example, 10 to 30 μm. 
     The positive electrode active material layer  31  is preferably formed over the entire region of each of the two surfaces of the positive electrode collector  30  other than at least one plain portion  32  which will be described later. The positive electrode active material layer  31  preferably contains a positive electrode active material, an electrically conductive agent, and a binding agent. The positive electrode  11  (positive electrode plate) may be formed in such a way that after a positive electrode mixture slurry containing the positive electrode active material, the electrically conductive agent, the binding agent, and a solvent, such as N-metnyl-2-pyrrolidone (NMP), is applied on the two surfaces of the positive electrode collector  30 , the coating films thus formed are then compressed. 
     As the positive electrode active material, a lithium transition metal oxide containing a transition metal element, such as Co, Mn, or Ni, may be mentioned by way of example. Although the lithium transition metal oxide is not particularly limited, a composite oxide represented by the general formula Li 1+x MO 2  (in the formula, −0.2&lt;x≤0.2 holds, and M represents at least one of Ni, Co, Mn, and Al) is preferable. 
     As an example of the electrically conductive agent, a carbon material, such as carbon black (CB), acetylene black (AB), Ketjen black, graphite, or the like may be mentioned. As an example of the binding agent, a fluorine-based resin, such as a polytetrafluoroethylene (PTFE) or a poly(vinylidene fluoride) (PVdF), a polyacrylonitrile (PAN), a polyimide (PI), an acrylic-based resin, an olefin-based resin, or the like may be mentioned. In addition, those resins each may be used together with a carboxymethyl cellulose (CMC) or its salt, a poly(ethylene oxide) (PEO), or the like. Those resins may be used alone, or at least two types thereof may be used in combination. 
     As described above, the positive electrode  11  includes the positive electrode lead  19  bonded to the positive electrode collector  30 . One end side portion (upper end side portion) of the positive electrode lead  19  extends past an upper end of the electrode group and is welded to the filter  22  of the sealing body  16 . On the other hand, the other end side portion (lower end side portion) of the positive electrode lead  19  is disposed on the positive electrode collector  30  and is welded to one surface of the collector. Since the width of the positive electrode collector  30  is smaller than the width of the negative electrode collector  35 , a root portion of the extension portion P 1  of the positive electrode lead  19  extending past one end (upper end) of the positive electrode collector  30  in a width direction faces the negative electrode  12  with the separator  13  interposed therebetween. 
     The positive electrode  11  has the plain portion  32  at which a surface of a metal forming the collector is exposed. The plain portion  32  is a portion to which the positive electrode lead  19  is connected and is a portion at which the surface of the positive electrode collector  30  is not covered with the positive electrode active material layer  31 . The plain portion  32  is formed to have a width larger than that of the positive electrode lead  19 . The plain portions  32  are preferably provided on two surfaces of the positive electrode  11  so as to be overlapped with each other in a thickness direction of the positive electrode  11 . 
     In the example shown in  FIG. 3 , at the central portion of the positive electrode  11  in the longitudinal direction, the plain portion  32  is provided over the entire length of the collector in the width direction. Although the plain portion  32  may be formed at an end portion side of the positive electrode  11  in the longitudinal direction, in view of the current collection, the plain portion  32  is preferably provided at a position equally apart from each of the two end portions in the longitudinal direction. In addition, the plain portion  32  may be provided to have a length from the upper end of the positive electrode  11  to a position located above the lower end thereof. The plain portion  32  is provided, for example, by intermittent application in which the positive electrode mixture slurry is not applied on a part of the positive electrode collector  30 . 
     The negative electrode  12  includes the belt-shaped negative electrode collector  35  and at least one negative electrode active material layer  36  formed on the negative electrode collector. In this embodiment, the negative electrode active material layers  36  are formed on two surfaces of the negative electrode collector  35 . For the negative electrode collector  35 , for example, foil of a metal, such as copper, or a film having a surface layer on which the metal mentioned above is disposed may be used. The thickness of the negative electrode collector  35  is, for example, 5 to 30 μm. 
     The negative electrode active material layers  36  are preferably formed over the entire regions of the two surfaces of the negative electrode collector  35  other than plain portions  37   a  and  37   b . The negative electrode active material layer  36  preferably contains a negative electrode active material and a binding agent. The negative electrode  12  (negative electrode plate) may be formed, for example, in such a way that after a negative electrode mixture slurry containing the negative electrode active material, the binding agent, water, and the like is applied on the two surfaces of the negative electrode collector  35 , the coating films thus formed are compressed. 
     As the negative electrode active material, any material capable of reversibly occluding and releasing lithium ions may be used, 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, an alloy of the metal mentioned above, or a composite oxide. As the binding agent contained in the negative electrode active material layer  36 , for example, a resin similar to that used in the case of the positive electrode  11  may be used. When the negative electrode mixture slurry is prepared using an aqueous solvent, a styrene-butadiene rubber (SBR), a CMC or its salt, a polyacrylic acid or its salt, a poly(vinyl alcohol), or the like may be used. Those materials may be used alone, or at least two thereof may be used in combination. 
     The negative electrode  12  has the plain portions  37   a  and  37   b  at each of which a surface of a metal forming the collector is exposed. The plain portions  37   a  and  37   b  are portions to which the negative electrode leads  20   a  and  20   b  are connected, respectively, and are portions at each of which the surface of the negative electrode collector  35  is not covered with the negative electrode active material layer  36 . The plain portions  37   a  and  37   b  are each formed to have a width larger than that of the corresponding negative electrode lead. The plain portions  37   a  are preferably provided on two surfaces of the negative electrode  12  so as to be overlapped with each other in a thickness direction of the negative electrode  12  (the same may also be applied to the plain portions  37   b ). 
     In the example shown in  FIG. 3 , at the two end portions of the negative electrode  12  in the longitudinal direction, the plain portions  37   a  and  37   b  are respectively provided over the entire length of the collector in the width direction. Although one of the plain portions  37   a  and  37   b  may be provided at a central portion side of the negative electrode collector  35  in the longitudinal direction, in view of the current collection, the plain portions are preferably separately provided at the two end portions in the longitudinal direction. In addition, the plain portions  37   a  and  37   b  each may also be formed to have a length from the lower end of the negative electrode  12  to a position located below the upper end thereof. The plain portions  37   a  and  37   b  are each provided, for example, by intermittent application in which the negative electrode mixture slurry is not applied on a part of the negative electrode collector  35 . 
     As the separator  13 , a porous sheet having ion permeability and an insulating property is used. As a particular example of the porous sheet, for example, a fine porous thin film, a woven cloth, or a non-woven cloth may be mentioned. As a material of the separator  13 , an olefin resin, such as a polyethylene or a polypropylene, is preferable. The thickness of the separator  13  is, for example, 10 to 50 Mm. The thickness of the separator  13  tends to be decreased in association with an increase in capacity and an increase in output of the battery. The separator  13  has, for example, a melting point of approximately 130° C. to 180° C. Hence, when heat generation occurs at the extension portion P 1  of the positive electrode lead  19  due to an external short circuit or the like, a portion of the separator  13  facing the extension portion P 1  may be melted in some cases. 
     As described above, the nonaqueous electrolyte secondary battery  10  has the insulating tape  40  adhered to, of the extension portion P 1  of the positive electrode lead  19  extending past the upper end of the positive electrode collector  30 , at least a facing region S 1  which is a range facing the negative electrode  12  with the separator  13  interposed therebetween. Since the extension portion P 1  of the positive electrode lead  19  is not in contact with the positive electrode collector  30  or the like, heat is not likely to be dissipated when heat generation occurs by an external short circuit or the like, and hence, the temperature is liable to increase. Since the root portion of the extension portion P 1  faces the negative electrode  12  with the separator  13  interposed therebetween, it is worried that an internal short circuit caused by melting of the separator  13  may be generated in some cases. The insulating tape  40  functions to suppress the internal short circuit as described above. 
     The insulating tape  40  has, for example, an approximately square shape in front view. The shape of the insulating tape  40  is not particularly limited when the tape can be adhered to the entire region of the facing region S 1 . In addition, since the positive electrode  11  is sandwiched with the negative electrode  12  from the two sides in the radial direction β of the electrode assembly  14 , the number of the facing regions S 1  of the positive electrode lead  19  is two. The insulating tapes  40  are adhered to the facing region S 1  facing the winding core side of the electrode assembly  14  and the facing region S 1  facing the winding outer side of the electrode assembly  14 . Although one insulating tape  40  may be wound around the root portion of the extension portion P 1 , the two insulating tapes  40  are preferably adhered to the respective facing regions S 1 . For the two insulating tapes  40 , for example, tapes equivalent to each other are used. 
     In this embodiment, the two insulating tapes  40  each protrude from the facing region S 1  to the two sides of the positive electrode lead  19  in the width direction, and those protrusion portions are bonded to each other. Hence, at the root portion of the extension portion P 1 , the side surfaces of the positive electrode lead  19  along a thickness direction are also covered with the insulating tapes  40 . After the two insulating tapes  40  are adhered to each other, for example, so as to cover at least the range of the facing regions S 1  and the side surfaces of the root portion of the extension portion P 1 , the positive electrode lead  19  is welded to the plain portion  32  of the positive electrode collector  30 . 
     The insulating tape  40  is preferably adhered not only to the facing region S 1  of the positive electrode lead  19  but also to the periphery thereof in consideration of winding misalignment of each electrode and the like of the electrode assembly  14 . The insulating tape  40  is adhered to the surface of the positive electrode lead  19  facing the winding core side so as to extend past a position facing the upper end of the negative electrode  12 . Furthermore, the insulating tape  40  may also be adhered so as to extend past a position facing the upper end of the separator  13 . In addition, the insulating tape  40  may be adhered to extend past the lower end of the extension portion P 1  to a non-extension portion P 2  disposed on the positive electrode collector  30 . As for the surface of the positive electrode lead  19  facing the winding outer side, the insulating tape  40  is also adhered to a range similar to that described above. The lower portion of the insulating tape  40  adhered to the surface of the positive electrode lead  19  facing the winding outer side is disposed between the non-extension portion P 2  of the positive electrode lead  19  and the positive electrode collector  30 . 
       FIG. 5  is a cross-sectional view of the insulating tape  40 . As illustrated in  FIG. 5 , the insulating tape  40  includes the base material layer  41 , the adhesive layer  42 , and the inorganic particle-containing layer  43  formed between the base material layer  41  and the adhesive layer  42 . The inorganic particle-containing layer  43  contains 20 percent by weight or more of inorganic particles with respect to the weight of the layer described above. When the content of the inorganic particles in the inorganic particle-containing layer  43  is less than 20 percent by weight, a sufficient heat resistance to prevent the internal short circuit caused by melting of the separator  13  cannot be obtained. The insulating tape  40  having the three-layer structure as described above is excellent in heat resistance and has a high piercing strength (mechanical strength). In this case, the “heat resistance” means properties in which the tape is difficult to be deteriorated and deformed by heat. 
     The content of the inorganic particles of the insulating tape  40  with respect to the weight of the insulating tape  40  other than the adhesive layer  42 , that is, with respect to the total weight of the base material layer  41  and the inorganic particle-containing layer  43 , is preferably less than 20 percent by weight, more preferably 10 percent by weight or less, and particularly preferably 5 to 10 percent by weight. As described above, when the addition amount of the inorganic particles is increased in the tape having a two-layer structure as disclosed in Patent Document 1, although the heat resistance is improved, the piercing strength is degraded. That is, the heat resistance and the piercing strength have a trade-off relationship. The insulating tape  40  is designed to decrease the content of the inorganic particles in the entire tape while the content of the inorganic particles is increased in the inorganic particle-containing layer  43 . According to the insulating tape  40  as described above, an excellent heat resistance and a high piercing strength can be simultaneously obtained. 
     The thickness of the insulating tape  40  is, for example, 20 to 70 μm and preferably 25 to 60 μm. The thickness of each layer of the insulating tape  40  can be measured by cross-sectional observation using a scanning electron microscope (SEM). The insulating tape  40  may have a layered structure including at least four layers. For example, the base material layer  41  is not limited to a monolayer structure and may be a laminate film formed of at least two layers equivalent to or different from each other. 
     The base material layer  41  preferably contains no inorganic particles and is preferably formed substantially only from an organic material. The rate of the organic material to the constituent materials of the base material layer  41  may be, for example, 90 percent by weight or more, preferably 95 percent by weight or more, or approximately 100 percent by weight. The primary component of the organic material is preferably a resin excellent in insulating property, electrolyte liquid resistance, heat resistance, piercing strength, and the like. The thickness of the base material layer  41  is, for example, 10 to 45 μm and preferably 15 to 35 μm. The thickness of the base material layer  41  is preferably larger than that of each of the adhesive layer  42  and the inorganic particle-containing layer  43  and is 50% or more of the thickness of the insulating tape  40 . 
     As a preferable resin forming the base material layer  41 , for example, there may be mentioned an ester-based resin, such as a poly(ethylene terephthalate) (PET), a polypropylene (PP), a polyimide (PI), a poly(phenylene sulfide), or a polyimide. Those resins may be used alone, or at least two types thereof may be used in combination. Among those resins mentioned above, a polyimide having a high piercing strength is particularly preferable. For the base material layer  41 , for example, a resin film containing a polyimide as a primary component may be used. 
     The adhesive layer  42  is a layer which imparts to the insulating tape  40 , an adhesion property to the positive electrode lead  19 . The adhesive layer  42  is formed, for example, by applying an adhesive on one surface of the base material layer  41  on which the inorganic particle-containing layer  43  is formed. As is the case of the base material layer  41 , the adhesive layer  42  is preferably formed using an adhesive (resin) excellent in insulating property, electrolyte liquid resistance, and the like. Although an adhesive forming the adhesive layer  42  may be either a hot-melt type which exhibits an adhesion property by heating or a thermosetting type which is cured by heating, in view of the productivity and the like, an adhesive having an adhesion property at room temperature is preferable. The adhesive layer  42  is formed, for example, using an acrylic-based adhesive or a synthetic rubber-based adhesive. The thickness of the adhesive layer  42  is, for example, 5 to 30 μm. 
     As described above, the inorganic particle-containing layer  43  is a layer containing 20 percent by weight or more of inorganic particles and is a layer mainly imparting a heat resistance to the insulating tape  40 . The inorganic particle-containing layer  43  preferably has a layer structure in which the inorganic particles are dispersed in a resin matrix which forms the layer. The inorganic particle-containing layer  43  is formed, for example, by applying a resin solution containing the inorganic particles to one surface of the base material layer  41 . The thickness of the inorganic particle-containing layer  43  is, for example, 0.5 to 10 μm and preferably 1 to 5 μm. 
     The content of the inorganic particles with respect to the weight of the inorganic particle-containing layer  43  is preferably 25 to 80 percent by weight, more preferably 30 to 80 percent by weight, and particularly preferably 35 to 80 percent by weight. In the insulating tape  40 , since the base material layer  41  is provided, and in addition, the inorganic particle-containing layer  43  is provided between the base material layer  41  and the adhesive layer  42 , even when the addition amount of the inorganic particles of the inorganic particle-containing layer  43  is increased, a preferable piercing strength can be secured. However, when the addition amount of the inorganic particles is excessively increased, the film strength of the inorganic particle-containing layer  43  is decreased, and the piercing strength may be decreased in some cases; hence, the upper limit of the content of the inorganic particles of the inorganic particle-containing layer  43  is preferably 80 percent by weight. In addition, the upper limit described above is further preferably 50 percent by weight. 
     As is the case of the base material layer  41 , a resin forming the inorganic particle-containing layer  43  is preferably excellent in insulating property, electrolyte liquid resistance, and the like, and in addition, is also preferably excellent in adhesion property to the inorganic particles and the base material layer  41 . As a preferable resin, for example, there may be mentioned an acrylic-based resin, a urethane-based resin, or an elastomer thereof. Those resins may be used alone, or at least two types thereof may be used in combination. 
     The inorganic particles forming the inorganic particle-containing layer  43  are preferably particles having an insulating property and a small particle diameter. The average particle diameter of the inorganic particles is, for example, 50 to 500 nm and preferably 50 to 200 nm. As preferable inorganic particles, for example, there may be mentioned titania (titanium oxide), alumina (aluminum oxide), silica (silicon oxide), or zirconia (zirconium oxide). Those types of inorganic particles may be used alone, or at least two types thereof may be used in combination. Among those inorganic particles, silica is particularly preferable. 
     EXAMPLES 
     Hereinafter, although the present disclosure will be further described with reference to Examples, the present disclosure is not limited thereto. 
     Example 1 
     [Formation of Positive Electrode] 
     After 100 parts by weight of a lithium transition metal oxide (average particle diameter: 12 μm) represented by LiNi 0.8 Co 0.15 Al 0.05 O 2  as a positive electrode active material, 2 parts by weight of acetylene black, and 2 parts by weight of poly(vinylidene fluoride) were mixed together, an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added thereto, so that a positive electrode mixture slurry was prepared. Next, the positive electrode mixture slurry was applied on two surfaces of a positive electrode collector formed of aluminum foil, and the coating films thus formed were dried. The collector on which the coating films were formed was compressed using a roller machine and then cut into a predetermined electrode size, so that a positive electrode plate in which the positive electrode active material layers were formed on the two surfaces of the positive electrode collector was formed. The length, the width, and the thickness of the positive electrode collector were 667 mm, 57 mm, and 15 μm, respectively. A plain portion to which a positive electrode lead is to be welded was provided at a central portion of the positive electrode plate in a longitudinal direction. 
     Insulating tapes each having a three-layer structure including a base material layer/an inorganic particle-containing layer/an adhesive layer were prepared, and the tapes described above were each adhered to a range of a root portion of an extension portion of the positive electrode lead and the periphery of the range. The two insulating tapes were adhered on two surfaces of the positive electrode lead so that the end portions of the tapes protruded from the two ends of the lead in a width direction. In addition, the portions of the tapes protruding from the lead were bonded to each other. The positive electrode lead to which the insulating tapes were adhered was welded to the plain portion of the collector, so that a positive electrode was formed. 
     A particular layered structure of the above insulating tape is as described below. 
     As the base material layer, a resin film (thickness: 25 μm) containing a polyimide as a primary component was used. The inorganic particle-containing layer had a layer structure in which silica particles in an amount of 25 percent by weight were dispersed in an acrylic resin. The thickness of the inorganic particle-containing layer was 1 μm. The adhesive layer was formed of an adhesive (primary component: acrylic-based resin) having an adhesion property at room temperature. The content of the silica particles with respect to the total weight of the base material layer and the inorganic particle-containing layer was 0.8 percent by weight. 
     [Formation of Negative Electrode] 
     After 100 parts by weight of a graphite powder (average particle diameter: 20 μm), 1 part by weight of a poly(vinylidene fluoride), and 1 part by weight of a carboxymethyl cellulose were mixed together, an appropriate amount of water was further added thereto, so that a negative electrode mixture slurry was prepared. Next, the negative electrode mixture slurry was applied on two surfaces of a negative electrode collector formed of copper foil, and the coating films thus formed were dried. The collector on which the coating films were formed was compressed using a roller machine and then cut into a predetermined electrode size, so that a negative electrode plate in which the negative electrode mixture layers were formed on the two surfaces of the negative electrode collector was formed. The length, the width, and the thickness of the negative electrode collector were 745 mm, 58.5 mm, and 8 μm, respectively. A plain portion was provided at a winding-finish side end portion of the negative electrode plate, and a negative electrode lead was welded to the plain portion, so that a negative electrode was formed. 
     [Preparation of Nonaqueous Electrolyte] 
     Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed together at a volume ratio of 4:6. In this mixed solvent, LiPFE was dissolved at a concentration of 1 mol/L, so that a nonaqueous electrolyte was prepared. 
     [Formation of Battery] 
     The positive electrode and the negative electrode were spirally wound with separators interposed therebetween, the separators each being formed of a polyethylene-made porous film (thickness: 16 μm), so that a wound electrode assembly was formed. In the electrode assembly thus obtained, the insulating tapes were each adhered to, of the extension portion of the positive electrode lead, a range facing the negative electrode with the separator interposed therebetween and the periphery of the range. After the electrode assembly was received in a cylindrical metal-made case main body having a bottom plate, an upper end portion of the positive electrode lead was welded to a filter of a sealing body, and a lower end portion of the negative electrode lead was welded to a bottom inner surface of the case main body. In addition, the nonaqueous electrolyte liquid was charged into the case main body, and an opening portion of the case main body was sealed by the sealing body, so that a cylindrical battery having a rated capacity of 3,350 mAh was formed. 
     Example 2 
     Except for that an insulating tape was used in which instead of the inorganic particle-containing layer of Example 1, an inorganic particle-containing layer containing 35 percent by weight of silica particles and having a thickness of 5 μm was formed, a positive electrode and a cylindrical battery were formed in a manner similar to that of Example 1. The content of the inorganic particles with respect to the total weight of the base material layer and the inorganic particle-containing layer was 5 percent by weight. 
     Comparative Example 1 
     Except for that an insulating tape including no inorganic particle-containing layer (the remaining layer structure was the same as that of Example 1) was used, a positive electrode and a cylindrical battery were formed in a manner similar to that of Example 1. 
     Comparative Example 2 
     Except for that an insulating tape was used in which instead of the inorganic particle-containing layer of Example 1, an inorganic particle-containing layer containing 10 percent by weight of silica particles and having a thickness of 5 μm was formed, a positive electrode and a cylindrical battery were formed in a manner similar to that of Example 1. The content of the inorganic particles with respect to the total weight of the base material layer and the inorganic particle-containing layer was 1.5 percent by weight. 
     Comparative Example 3 
     Except for that an insulating tape having a two-layer structure which included an inorganic particle-containing layer, an adhesive layer, and no base material layer was used, a positive electrode and a cylindrical battery were formed in a manner similar to that of Example 1. The content of silica particles in the inorganic particle-containing layer was set to 50 percent by weight, and the thickness of the inorganic particle-containing layer was set to 25 μm. 
     A piercing test was performed on each of the insulating tapes of the above Examples and Comparative Examples by the following method. In addition, an external short-circuit test was performed on each battery by the following method. 
     [Piercing Test] 
     The surface of each of the above insulating tapes was pierced with a needle, and a pressing force (N) at which penetration was confirmed by visual inspection was measured. The pressing force is shown in Table 1 as a piercing strength. A higher pressing force indicates a higher piercing strength of the tape. 
     [External Short-Circuit Test] 
     A pretreatment was performed on each battery under the following conditions. 
     Discharge (CC): 3,350 mA×2.5 V, 550 mA×2.5 V 
     Discharge Rest: 20 minutes 
     Charge (CCCV): 1,675 mA×4.25 V, 67 mA cut 
     Charge Rest: 20 minutes 
     An external short-circuit test was performed on each of the batteries processed by the above pretreatment. 
     External Short-Circuit Resistance: 20 mΩ or less 
     Test temperature: 60° C. 
     The maximum reaching temperature (battery side surface temperature) of the battery was measured using a thermocouple, and the measurement results are shown in Table 1. When the above temperature is lower, it indicates that the internal short circuit induced by an external short circuit is more unlikely to occur. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 EXAMPLE  
                 EXAMPLE  
                 COMPARATIVE  
                 COMPARATIVE  
                 COMPARATIVE  
               
               
                   
                 1  
                 2  
                 EXAMPLE 1  
                 EXAMPLE 2  
                 EXAMPLE 3 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 THICKNESS OF BASE  
                 25  
                 25  
                 25  
                 25  
                 — 
               
               
                 MATERIAL LAYER (μm)  
                   
                   
                   
                   
                   
               
               
                 THICKNESS OF INORGANIC  
                 1  
                 5  
                 — 
                 5  
                 25  
               
               
                 PARTICLE-CONTAINING  
                   
                   
                   
                   
                   
               
               
                 LAYER (μm)  
                   
                   
                   
                   
                   
               
               
                 CONTENT OF INORGANIC  
                 25  
                 35  
                 0  
                 10  
                 50  
               
               
                 PARTICLES* 1    
                   
                   
                   
                   
                   
               
               
                 CONTENT OF INORGANIC  
                 0.8  
                 5  
                 0  
                 1.5  
                 50  
               
               
                 PARTICLES* 2    
                   
                   
                   
                   
                   
               
               
                 PIERCING STRENGTH (N)  
                 11.1  
                 11.3  
                 10.9  
                 11.4  
                 7.3  
               
               
                 MAXIMUM REACHING  
                 122  
                 120  
                 142  
                 137  
                 120  
               
               
                 TEMPERATURE (° C.) 
               
               
                   
               
               
                 * 1 Content (percent by weight) of the inorganic particles with respect to the weight of the inorganic particle-containing layer. 
               
               
                 * 2 Content (percent by weight) of the inorganrc particles with respect to the weight of the insulating tape other than the adhesive layer. 
               
            
           
         
       
     
     As shown in Table 1, compared to the batteries of Comparative Examples 1 and 2, according to the batteries of Examples 1 and 2, the maximum reaching temperature in the external short-circuit test is low, and the internal short circuit induced by an external short circuit is suppressed. In the above external short-circuit test, in every battery, a large current flows through the positive electrode lead, and heat generation occurs at the extension portion, so that the separator is melted by this heat. However, according to the batteries of Examples 1 and 2, it is believed that since the contact between the positive electrode lead and the negative electrode is prevented by an insulating tape having a high heat resistance, the internal short circuit is suppressed. On the other hand, according to the batteries of Comparative Examples 1 and 2, it is believed that since the heat resistance of the insulating tape is not sufficient, the contact between the positive electrode lead and the negative electrode cannot be prevented, and as a result, the battery temperature is remarkably increased. 
     Furthermore, since the piercing strength of the insulating tape of each of Examples 1 and 2 is high, according to the battery of each of Examples 1 and 2 using the insulating tape described above, the internal short circuit caused by intrusion of an electrically conductive foreign material between the extension portion of the positive electrode lead and the negative electrode can be highly suppressed. On the other hand, since the insulating tape of Comparative Example 3 has a low piercing strength although having a high heat resistance, according to the battery of Comparative Example 3 using the insulating tape described above, the internal short circuit caused by an electrically conductive foreign material cannot be sufficiently overcome. 
     That is, only in the case in which the insulating tape including at least three layers, that is, the base material layer/the inorganic particle-containing layer containing 20 percent by weight or more of inorganic particles/the adhesive layer, is used, the internal short circuit induced by an external short circuit and the internal short circuit caused by an electrically conductive foreign material can both be highly suppressed. 
     REFERENCE SIGNS LIST 
     nonaqueous electrolyte secondary battery,  11  positive electrode,  12  negative electrode,  13  separator,  14  electrode assembly,  15  case main body,  16  sealing body,  17 ,  18  insulating plate,  19  positive electrode lead,  20   a ,  20   b  negative electrode lead,  21  protruding portion,  22  filter,  23  lower valve body,  24  insulating member,  25  upper valve body,  26  cap,  27  gasket,  28  space,  30  positive electrode collector,  31  positive electrode active material layer,  32  plain portion,  35  negative electrode collector,  36  negative electrode active material layer,  37   a ,  37   b  plain portion,  40  insulating tape,  41  base material layer,  42  adhesive layer,  43  inorganic particle-containing layer, P 1  extension portion, P 2  non-extension portion, S 1  facing region