JP2008130370A - Secondary battery for large-current discharge - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a secondary battery for large-current discharge capable of exerting a high output even at large-current discharge and securing safety at abnormalities of the battery. <P>SOLUTION: For a lithium-ion secondary battery 20, two sheets of films 1, 1' having polypropylene (PP) films of a melting layer are used for a battery container. An electrode group 4 is sealed between the films 1, 1'. The films 1, 1' are provided with a melting part 9 with the PP films thermally melted with each other. At two opposite sides of the films 1, 1', a cathode terminal 2 and an anode terminal 3 each are arranged. At one side other than the two with the cathode terminal 2 and the anode terminal 3 arranged, respectively, a resin film 11 is pinched in and thermally melted. As the resin film 11, a PE film with a softening temperature lower than that of the PP film of the melting layer is used. At abnormalities of the battery, the resin film 11 is softened earlier than the PP films of the melting layer to generate a gap at the melting part 9. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a secondary battery for large current discharge, and more particularly to a secondary battery for large current discharge in which an electrode group is housed in a film-like sealed container having a welded portion in which heat weldable films are thermally welded to each other.

  Conventionally, an assembled battery in which a plurality of secondary batteries are connected in series or in series-parallel has been used for a large-current charge / discharge power source such as an electric vehicle or a hybrid vehicle. Generally, in such a large current charging / discharging power source, for example, 40 to 100 cylindrical secondary batteries are used.

  In order to reduce costs, an iron-based material is generally used for a cylindrical container used for a cylindrical secondary battery. However, since iron has a large specific gravity, it has been a major limitation in increasing the energy efficiency per unit weight of the battery (to reduce weight). This problem is also common in small sealed lead batteries using injection molded resin containers, and it is difficult to increase the energy efficiency per volume due to the thickness, as well as the weight is not so light. There was a problem. For this reason, a film-type secondary battery using a so-called laminate film (hereinafter simply referred to as a film) in which an aluminum foil or the like is incorporated in the inner layer as a gas barrier layer and a heat-weldable film is used as a heat-welded layer as a battery container. A technique of (laminate cell) is disclosed (for example, see Patent Document 1). In such a film-type secondary battery, an electrode group is housed in a film-like sealed container having a welded portion where heat-weldable films are heat-welded.

  In addition, in the case of a secondary battery, if it becomes overcharged due to a failure or misuse of the charging device, or if it is used in a high temperature environment, the secondary battery becomes abnormal and the temperature rises. The internal pressure of the battery rises due to the gas generated by vaporization or decomposition of the electrolyte accompanying this. In preparation for such a case, a cylindrical secondary battery has a current interrupting mechanism (safety mechanism) that cuts off the current by breaking the joint due to an increase in internal pressure by forming a weak joint inside the battery. Incorporated. However, since the film type secondary battery has a very simple structure, it is difficult to incorporate such a current interruption mechanism inside the battery. For this reason, when the secondary battery expands due to an increase in internal pressure when the battery is abnormal, the welded portion of the film is peeled off and gas is ejected outside the battery. When the positive and negative electrodes are short-circuited due to this heat generation and gas ejection, there is a problem that rapid heat generation occurs due to a short-circuit current, causing a thermal runaway reaction of the active material (rapid exothermic reaction with a large amount of gas generation).

  In order to solve this, the positive electrode terminal and the negative electrode terminal are taken out from one side of the rectangular shape, and a resin film having a melting point lower than that of the heat weldable film is sandwiched between the positive electrode terminal and the negative electrode terminal and thermally welded. Further, a technique of a film type secondary battery is disclosed (for example, see Patent Document 2). In addition, when a low melting point resin film is sandwiched in the welded portion, a technique for preventing the end portion of the resin film from being sandwiched by the boundary of the welded portion is also disclosed in order to prevent the electrolytic solution from adhering to the resin film. (See Patent Document 3). In these techniques, a fragile portion is formed in the welded portion by sandwiching a low melting point resin film. For this reason, when the temperature and the internal pressure rise when the battery is abnormal, the low melting point resin film is melted by heat and a gas discharge path is formed, so that the internal pressure can be reduced.

JP-A-60-230354 JP 2001-93489 A JP 2005-276700 A

  However, in the technique of Patent Document 2, since the resin film is located between the positive electrode terminal and the negative electrode terminal, along with the increase in the internal pressure of the battery due to the thermal melting of the resin film, the positive electrode terminal and There is a problem that the negative electrode terminal is short-circuited and is likely to shift to a thermal runaway reaction. In the techniques of Patent Documents 2 and 3, since the positive electrode terminal and the negative electrode terminal are derived from one side of the rectangular shape, the distance from the electrode group to the terminal is increased and the internal resistance is increased. Therefore, it is not suitable as a secondary battery used for a power source for a hybrid vehicle in which a large current discharge is indispensable when starting the engine.

  In view of the above-described case, an object of the present invention is to provide a secondary battery for large current discharge that can exhibit high output even during large current discharge and can ensure safety when the battery is abnormal.

  In order to solve the above problems, the present invention provides a secondary battery for large current discharge in which an electrode group is housed in a film-like sealed container having a welded portion in which heat-welding films are thermally welded to each other. The extracted positive electrode terminal and negative electrode terminal are respectively disposed on two opposite sides of the sealed container, and a resin film that is softened at a temperature lower than that of the heat-weldable film is sandwiched between at least a part of the welded portions. It is characterized by being thermally welded.

  In the present invention, the positive electrode terminal and the negative electrode terminal derived from the electrode group are respectively disposed on two opposite sides of the sealed container, and a conductive path from the electrode group to the terminal is formed at the shortest distance. Since the resistance is small and high output can be achieved even during large current discharge, a resin film that softens at a temperature lower than that of the heat-weldable film is sandwiched and heat-sealed at least in part of the welded part. When the temperature and internal pressure rise, before the pressure limit of the closed container is reached, a release path is formed where the resin film melts and releases the internal pressure, thus reducing the internal pressure and gently terminating the battery function. It is possible to ensure safety when the battery is abnormal.

  In this case, if the softening temperature of the resin film is lower by 40 ° C. or more than the softening temperature of the heat-weldable film, the resin film can be reliably melted by heat before reaching the pressure limit of the sealed container. The resin film may be a polyolefin resin. Also, if the resin film is sandwiched and thermally welded between the welded portions other than the two sides where the positive electrode terminal and the negative electrode terminal are respectively disposed, thermal welding is performed at the welded portion where the positive electrode terminal and the negative electrode terminal are disposed. Since the hermeticity is maintained by heat welding between the films, the battery function during normal charging and discharging can be ensured.

  According to the present invention, the positive electrode terminal and the negative electrode terminal are respectively disposed on two opposite sides of the sealed container, and a conductive path from the electrode group to the terminal is formed at the shortest distance, so that the internal resistance is small and large. A high output can be exhibited even during current discharge, and a resin film that is softened at a temperature lower than that of the heat-weldable film is sandwiched and heat-welded in at least a part of the welded portion. Since a release path for releasing the internal pressure by heat melting is formed, it is possible to obtain an effect that the internal pressure can be reduced, the battery function can be terminated gently, and safety can be ensured.

  Hereinafter, an embodiment in which the present invention is applied to a lithium ion secondary battery used for a power source for a hybrid vehicle using a laminate film as a battery container will be described with reference to the drawings. The lithium ion secondary battery of this embodiment is used as a battery module in which a plurality are connected in series or in series-parallel.

(Constitution)
As shown in FIG. 1, in the lithium ion secondary battery 20 of the present embodiment, two rectangular flexible film laminate films (hereinafter simply referred to as films) 1, 1 ′ are used for the battery container. Has been.

As shown in FIG. 2, the film 1 has the aluminum foil 1b used as a gas barrier layer in an inner layer. A polypropylene (hereinafter abbreviated as PP) film 1a as a heat-welding film to be a welding layer is bonded to one surface of the aluminum foil 1b, and a polyethylene terephthalate (hereinafter abbreviated as PET) is bonded to the other surface. The film 1c is bonded. In this example, the film 1 is set to a thickness of about 120 μm. Similar to the film 1, the film 1 ′ is composed of a PP film 1′a, an aluminum foil 1′b, and a PET film 1′c. As shown in FIG. 1, the film 1 ′ is a flat planar film, and the film 1 is a cup-shaped film having a substantially central portion formed into a convex shape. The films 1 and 1 ′ are disposed so that the PP films 1a and 1′a face each other. An electrode group 4 is disposed between the films 1 and 1 '. Two positive electrode terminals 2 and two negative electrode terminals 3 are arranged on two opposite sides of the films 1 and 1 ′, respectively, with their tip portions protruding outward in opposite directions. Film 1, 1 '
Has a welded portion 9 in which the PP films 1a, 1'a are thermally welded to each other at the peripheral portion. For this reason, the electrode group 4 is accommodated in a film-like hermetic container having a welded portion 9 at the periphery, and the lithium ion secondary battery 20 has a hermetically sealed structure. Each positive electrode terminal 2 and each negative electrode terminal 3 are sandwiched and welded to the welding part 9 via a sealing material 10.

  In addition, as shown in FIG. 2, a rectangular resin film 11 is sandwiched between the welded portion 9 and substantially the center of one of the sides other than the two sides where the positive electrode terminal 2 and the negative electrode terminal 3 are respectively disposed. Heat welded. For the resin film 11, a material whose softening temperature is lower than the softening temperature of the PP films 1a and 1'a is selected. That is, the resin film 11 has a lower melting point than the PP films 1a and 1'a. In this example, the softening temperature of the PP films 1a and 1′a is about 150 ° C., whereas the resin film 11 is a polyethylene (hereinafter abbreviated as PE) film having a softening temperature of about 80 ° C. Is used. In this example, the length of the resin film 11 in the longitudinal direction along the sides of the films 1 and 1 ′ is about 30 mm, and the size in the width direction perpendicular to the longitudinal direction is larger than the welding width of the welded portion 9. Is about 30 μm.

  In the electrode group 4, 19 positive plates and 20 negative plates are alternately stacked. Each positive electrode plate is inserted into a separator formed into a bag shape by heat welding. For example, a polyethylene porous film having a thickness of 25 μm and a width of 100 mm is used as the separator. The positive electrode plate and the negative electrode plate are overlapped so that the positive electrode terminal 2 and the negative electrode terminal 3 are led out in directions opposite to each other. The two positive terminals 2 and the two negative terminals 3 are arranged so as to be symmetric with respect to a center line M orthogonal to two opposing sides of the films 1, 1 ′.

  The positive electrode strap portion 5 formed integrally with the positive electrode terminal 2 uses an aluminum alloy A3003-H12 having a thickness of 0.3 mm, and the portion of the positive electrode terminal 2 that is not in contact with the electrolytic solution (exposed to the outside of the battery). The nickel plate having a thickness of 0.1 mm is clad on one side only. On the other hand, a copper plate C1020-1 / 2H having a thickness of 0.3 mm is used for the negative electrode strap portion 7 formed integrally with the negative electrode terminal 3, and only on both sides of the negative electrode terminal 3 exposed to the outside of the battery. A nickel plate having a thickness of 0.05 mm is clad. The positive electrode strap part 5 and the negative electrode strap part 7 are joined to the plain part 6 of the positive electrode current collector and the plain part 8 of the negative electrode current collector, respectively, by ultrasonic welding. The thickness of the electrode group 4 is approximately 4.8 mm.

  At the time of assembling the lithium ion secondary battery 20, the electrode group 4 is placed at a substantially central portion of the film 1, the film 1 'is placed on the upper side of the electrode group 4, and the four sides are thermally welded to form the welded portion 9. Is done. At this time, a predetermined amount of non-aqueous electrolyte is used by using a syringe from the mating surface of the films 1 and 1 ′ left without being thermally welded to a part of the side where the positive electrode terminal 2 and the negative electrode terminal 3 are not provided. Is injected. During the thermal welding of the mating surfaces, the resin film 11 is sandwiched and thermally welded. The four sides of the films 1, 1 ′ were sealed by heat welding to complete the lithium ion secondary battery 20. In this example, the welding width of the welding part 9 is set to about 10 mm over the entire circumference. Two positive electrode terminals 2 and two negative electrode terminals 3 are led out from two opposite sides of the battery container via a positive electrode strap portion 5 and a negative electrode strap portion 7, respectively.

  At the time of producing the positive electrode plate constituting the electrode group 4, a lithium transition metal double oxide such as lithium manganate as a positive electrode active material, carbon powder as a conductive material, and polyvinylidene fluoride as a binder are N as solvents. -A slurry is prepared by dispersing and mixing in methyl-2-pyrrolidone. This slurry is applied to both surfaces of a 20 μm thick aluminum foil as a positive electrode current collector, dried, pressed and integrated. At this time, a plain portion 6 on which no slurry is applied is formed on one side of the aluminum foil. Thereafter, it is cut into a width of 94 mm to produce a strip-like positive electrode plate. The width of the coated part is set to 86 mm, and the width of the plain part 6 is set to 10 mm.

  On the other hand, at the time of producing the negative electrode plate, carbon particles as a negative electrode active material and polyvinylidene fluoride as a binder are charged and mixed in N-methyl-2-pyrrolidone as a solvent to produce a slurry solution. . This slurry is applied to both sides of a 10 μm thick copper foil as a negative electrode current collector, dried, pressed and integrated. At this time, a plain portion 8 on which no slurry is applied is formed on one side of the copper foil. Then, it cut | disconnects to width 96mm and a strip-shaped negative electrode plate is produced. The width of the coating part is set to 88 mm, and the width of the plain part 8 is set to 10 mm.

(Action etc.)
Next, the operation and the like of the lithium ion secondary battery 20 of the present embodiment will be described.

  In the lithium ion secondary battery 20 of the present embodiment, since a film-like hermetic container composed of films 1 and 1 ′ is used as a battery container, compared with a battery using a battery container of an iron-based material, The weight of the battery can be reduced. Further, in the lithium ion secondary battery 20 of the present embodiment, the resin film 11 is sandwiched and thermally welded to the welded portion 9 where the PP films 1a and 1'a are thermally welded. As the resin film 11, a PE film having a softening temperature lower than that of the PP films 1a and 1'a is used. For this reason, the resin film 11 softens or melts faster than the PP films 1a and 1'a when the battery is abnormally accompanied by a temperature rise. When the resin film 11 is softened, the internal pressure of the battery is increased, so that a gap is generated in the welded portion 9 to form a discharge path for releasing gas generated in the battery (a release path for releasing the internal pressure of the battery). When gas starts to be released through this discharge path, coupled with the fact that the battery internal pressure is rising, the gap generated in the welded portion 9 is widened in the range of the length of the resin film 11 (film 1, 1 'is peeled off). Thereby, the release of the gas generated in the battery is promoted, so that the internal pressure of the battery can be reduced. Furthermore, since the heat of vaporization is lost due to volatilization of the non-aqueous electrolyte accompanying gas release, the battery temperature can be lowered. Therefore, in the lithium ion secondary battery 20, the battery function can be terminated gently, and the safety when the battery is abnormal can be ensured.

  Moreover, in the lithium ion secondary battery 20 of this embodiment, the positive electrode terminal 2 and the negative electrode terminal 3 are each arrange | positioned at the two opposing sides of the films 1 and 1 '. For this reason, even if the resin film 11 is softened and gas is released when the battery is abnormal, contact between the positive electrode terminal 2 and the negative electrode terminal 3 can be suppressed. Thereby, since an internal short circuit between the positive electrode terminal 2 and the negative electrode terminal 3 is suppressed, it is possible to suppress a transition to a thermal runaway reaction accompanied by a rapid increase in temperature and pressure, and to ensure safety in the event of battery abnormality. it can.

  Furthermore, in the lithium ion secondary battery 20 of the present embodiment, the positive electrode plate and the negative electrode plate constituting the electrode group 4 are stacked such that the positive electrode terminal 2 and the negative electrode terminal 3 are led out in opposite directions. Two terminals 2 and two negative terminals 3 are provided on each of two opposing sides of the films 1 and 1 '. For this reason, the conductive distance from the positive electrode plate and the negative electrode plate to the positive electrode terminal 2 and the negative electrode terminal 3 becomes the shortest, and the internal resistance can be reduced. Thereby, a high output can be secured by suppressing a decrease in output even during a large current discharge. Accordingly, the lithium ion secondary battery 20 of the present embodiment is excellent in safety in the event of battery abnormality and exhibits high output even during large current discharge. It can be suitably used as a secondary battery.

  Conventional lithium-ion secondary batteries with a battery container made of a film are produced by vaporization of non-aqueous electrolyte and generation of decomposition gas when the battery temperature abnormally rises when overcharged or when used in a high temperature environment. When the internal pressure rises, the heat welded part of the film is peeled off, and the generated gas is ejected outside the battery. At this time, when the positive electrode plate and the negative electrode plate are short-circuited, rapid heat generation occurs due to the short-circuit current, and the reaction shifts to a thermal runaway reaction, causing a violent reaction. In addition, when the positive electrode terminal and the negative electrode terminal are sandwiched between the same sides (the extraction direction of the positive electrode terminal and the negative electrode terminal is the same direction), the conductive path from the positive / negative electrode plate to the positive / negative electrode terminal becomes long, and the internal resistance Increases, which causes a decrease in output during large current discharge. The increase in battery temperature and internal pressure that occurs when the battery is abnormal tends to become more rapid (violent) as the battery capacity increases. In general, a secondary battery for discharging a large current has a larger battery capacity than a secondary battery for a small consumer, and thus it is extremely important to ensure safety when the battery is abnormal. In addition to this, when used for a hybrid vehicle power supply or the like, a large current (for example, 500 A or more) is required to discharge when starting the engine, and a large current charging is required during regeneration, reducing internal resistance. It is also important to ensure high output. The present embodiment is a lithium ion secondary battery 20 that can solve these problems.

  In the present embodiment, the example in which the resin film 11 is sandwiched between the welding portions 9 other than the two sides on which the positive electrode terminal 2 and the negative electrode terminal 3 are disposed has been described, but the present invention is not limited thereto. For example, it may be sandwiched between two positive terminals 2 or two negative terminals 3 and heat-sealed. Even if it does in this way, since the positive electrode terminal 2 and the negative electrode terminal 3 are arrange | positioned at 2 sides which oppose, even if the resin film 11 softens, the contact of the positive electrode terminal 2 and the negative electrode terminal 3 can be avoided. The effects described above can be obtained. Considering that the portion where the positive electrode terminal 2 and the negative electrode terminal 3 are disposed is required to have reliability that can withstand long-term use in normal charging / discharging, the portion sandwiching the resin film 11 includes the positive electrode terminal 2 and the negative electrode terminal. It is desirable to use other than the two sides where the terminals 3 are disposed. Moreover, in this embodiment, although the example which pinches | interposes the resin film 11 in one place of the welding part 9 was shown, you may make it pinch | pinch in two or more places. Further, the size of the resin film 11 is not particularly limited.

  Moreover, although PE film was illustrated as the resin film 11 in this embodiment, this invention is not limited to this, Softening of the heat-weldable film which comprises the welding layer whose softening temperature is the films 1 and 1 '. Any film lower than the temperature (low melting point) may be used. Considering that generally a PP film is used for the welding layers of the films 1 and 1 ′, it is preferable to use a film of a polyolefin resin such as PE or PP, for example.

  Furthermore, in this embodiment, although the film 1 and 1 'illustrated the film comprised by three layers, PP film 1a, 1'a / aluminum foil 1b, 1'b / PET film 1c, 1'c, The invention is not particularly limited to the structure of the film, and any film having a flexible structure that can be used as a battery container can be used. In the present embodiment, an example in which the rectangular films 1 and 1 ′ are used has been described, but the present invention is not limited to this. It suffices to have two sides facing each other in order to dispose the positive electrode terminal 2 and the negative electrode terminal 3, and the portions other than the two facing sides may be, for example, arcuate. Furthermore, in this embodiment, although the example which comprises a sealed container by using two films 1 and 1 'and heat-welding four sides was shown, the center part of the longitudinal direction was used using one film. You may make it return. In this case, the hermetic container can be configured by thermally welding the three sides.

  Furthermore, in the present embodiment, an example in which two positive terminals 2 and two negative terminals 3 are provided has been shown. However, the present invention is not limited to this, for example, one by one or three by three. It is good. In consideration of large current charging / discharging, it is preferable to arrange a plurality of currents in order to secure a current-carrying area. Further, in the present embodiment, the electrode group 4 in which the positive electrode plates and the negative electrode plates formed in a strip shape are alternately laminated is illustrated, but the present invention is not limited to this, and for example, the positive electrode plate formed in a belt shape And you may make it wind a negative electrode plate flatly.

  Furthermore, in the present embodiment, the lithium ion secondary battery 20 is exemplified, but the present invention is not limited to this, and can be applied to a secondary battery using a flexible film as a battery container. . Of course, there is no limitation on materials such as positive and negative electrode active materials used in the lithium ion secondary battery.

  Next, examples of the lithium ion secondary battery 20 manufactured according to the present embodiment will be described. In addition, it describes together about the lithium ion secondary battery of the comparative example manufactured for the comparison.

  As shown in Table 1 below, in Examples 1 to 3, the resin film 11 was replaced and a lithium ion secondary battery 20 was manufactured. As the resin film 11, a PE film having a softening temperature of about 80 ° C in Example 1, a PP film having a softening temperature of about 110 ° C in Example 2, and a PP film having a softening temperature of about 140 ° C in Example 3 are used. It was. On the other hand, in Comparative Example 1, a lithium ion secondary battery was manufactured in the same manner as in this embodiment except that the resin film 11 was not sandwiched. Therefore, Comparative Example 1 is a conventional lithium ion secondary battery. The softening temperature of the PP films 1a and 1'a is about 150 ° C.

(Safety evaluation)
For each 100 lithium ion secondary batteries 20 of each example and comparative example, the presence or absence of ignition (safety) when charging until overcharged at 5 V, 1 CA (3.2 A) was evaluated. The safety evaluation results are shown in Table 1 below. In Table 1, the low melting point film indicates the resin film 11.

  As shown in Table 1, in the lithium ion secondary battery of Comparative Example 1 in which no low melting point film was sandwiched, the internal pressure and temperature were about 110% charged (that is, about 10% overcharged) with respect to the theoretical capacity. Gradually began to rise, reaching about 230-260% charge, and all 100 tested tested ignited.

  On the other hand, in the lithium ion secondary battery 20 of Example 1 in which a PE film having a softening temperature of 80 ° C. was sandwiched, the internal pressure and temperature began to rise at the time of charging of about 110% as in Comparative Example 1, When about 150% charge was reached, the softening temperature of the PE film was reached, and a gap was formed in the heat welded portion 9. As a result, the internal gas was released and the temperature rise was suppressed. This is presumably because the non-aqueous electrolyte was rapidly vaporized and released outside the battery as the gas was released, and the heat of vaporization was taken away. Furthermore, although charging was continued for about 8 hours as it was, only a minute current continued to flow, and none of the lithium ion secondary batteries 20 ignited. This is because the internal resistance of the lithium ion secondary battery 20 suddenly increased due to the vaporization of the non-aqueous electrolyte, the terminal voltage finally reached the power supply voltage (5 V), and then the current value rapidly decreased. Conceivable. Further, in the lithium ion secondary battery 20 of Example 2 in which a PP film having a softening temperature of 110 ° C. was sandwiched, the internal pressure and temperature similarly increased, and when the battery reached about 170% charge, the PP film softened and was thermally welded. A gap was formed in the portion 9, and thereafter, the same process as that of the lithium ion secondary battery 20 of Example 1 was followed, and the ignition did not occur.

  On the other hand, in the lithium ion secondary battery 20 of Example 3 in which a PP film having a softening temperature of 140 ° C. was sandwiched, a gap was formed in the heat-welded portion 9 when the charging reached about 210%. About 79 of the 100 lithium ion secondary batteries 20 evaluated, the same process as that of the lithium ion secondary battery 20 of Examples 1 and 2 was followed, and no ignition occurred. The remaining 21 lithium ion secondary batteries 20 continued to rise in temperature, and reached ignition when they reached about 240 to 260% charge. This is because the difference in softening temperature between the resin film 11 and the PP films 1a and 1′a is less than 40 ° C., so the softening of the resin film 11 is delayed and the internal pressure is increased until just before the battery container reaches the pressure limit. This is probably because the temperature continued to rise. Therefore, it has been found that it is desirable that the softening temperature of the resin film 11 is 40 ° C. or less lower than the PP films 1a and 1′a (the difference in softening temperature is 40 ° C. or more).

  From the above evaluation results, the lithium ion secondary battery 20 of the present embodiment is excellent in safety without being ignited or exploding even if it is overcharged due to a failure of the charging system or the like. It was confirmed.

  The present invention provides a secondary battery for large current discharge that can exhibit high output even during large current discharge and can ensure safety in the event of battery abnormality. In order to contribute, it has industrial applicability.

It is the top view and side sectional view which removed a part of cup-shaped film of the lithium ion secondary battery of embodiment which applied this invention. It is sectional drawing which shows the positional relationship of the laminate film and resin film in the A-A 'cross section of FIG.

Explanation of symbols

1, 1 'Laminate film 1a, 1'a Polypropylene film (heat welding film)
2 Positive electrode terminal 3 Negative electrode terminal 4 Electrode group 9 Welding part 11 Resin film 20 Lithium ion secondary battery

Claims (4)

  In a secondary battery for large current discharge in which an electrode group is housed in a film-like sealed container having a welded portion in which heat-welding films are thermally welded, the positive electrode terminal and the negative electrode terminal derived from the electrode group are sealed. A resin film that is disposed at two opposite sides of the container and is softened at least at a part of the welded portion by sandwiching a resin film that softens at a temperature lower than that of the heat-weldable film. Next battery.   The secondary battery according to claim 1, wherein the softening temperature of the resin film is 40 ° C. or more lower than the softening temperature of the heat-weldable film.   The secondary battery according to claim 1, wherein the resin film is a polyolefin-based resin.   2. The secondary battery according to claim 1, wherein the resin film is thermally welded by being sandwiched between welding portions other than two sides where the positive electrode terminal and the negative electrode terminal are respectively disposed.

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