JP2006079932A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery not generating thermal runaway even by temperature increase caused by overcharge or the like. <P>SOLUTION: The lithium ion secondary battery comprises a cathode plate 11 and an anode plate 12 facing each other with an inter-electrode separator 13 insulating them from each other, and a short circuit foil 14 is arranged so as to face the anode plate 12. The short circuit foil 14 is composed of a conductive member 34 and a resistant film 35 fitted to both surfaces of the conductive member. Further, a low-melting-point separator 15 having a melting point lower than that of the inter-electrode separator 13, insulating the anode plate 12 from the short-circuit foil 14 is arranged between them. The cathode plate 11 is electrically connected to the conductive member 34 of the short circuit foil 14. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a chargeable / dischargeable lithium ion secondary battery. More specifically, the present invention relates to a lithium ion secondary battery in which a positive electrode plate and a negative electrode plate are wound or stacked with an inter-electrode separator interposed therebetween.

  Conventionally, there is a secondary battery in which a positive electrode plate coated with a positive electrode active material and a negative electrode plate coated with a negative electrode active material are wound or laminated with an interelectrode separator interposed therebetween. In particular, lithium ion secondary batteries that use lithium cobalt oxide or the like as the positive electrode active material are widely used for small consumer applications. In this lithium ion secondary battery, lithium ions are occluded or released between the active materials of both electrodes by charging and discharging. High-capacity, high-power batteries are generally used as lithium ion secondary batteries for automobiles.

  In such a lithium ion secondary battery, heat is generated along with charge and discharge. In particular, when an overcharge due to a system failure or a short-circuit failure due to an unexpected accident occurs, the battery temperature may increase significantly due to an exothermic reaction. As the temperature rises, the interelectrode separator melts or deforms, and there is a problem that thermal runaway may occur as shown in FIG. FIG. 9 shows the result of an overcharge test in which a conventional secondary battery is continuously charged with a charging current of 10A. In this figure, the solid line shows the temperature change, and the vertical axis is the left end side. A broken line indicates a change in voltage, and the vertical axis indicates the right end side. As shown in this figure, the separator between the electrodes began to melt at time t1, the temperature rose rapidly, and a thermal runaway state occurred at time t2.

On the other hand, a secondary battery in which a resin layer that melts at a lower temperature than the interelectrode separator is provided between the positive and negative electrodes has been proposed (see, for example, Patent Document 1 and Patent Document 2). According to these secondary batteries, the resin layer is melted before the separator between the electrodes is melted or deformed, and an internal short circuit is generated only at this portion. As a result, the energy of the battery is dissipated from this part, and the battery is prevented from reaching a thermal runaway reaction.
JP 2003-243037 A JP 2003-338315 A

  However, in the conventional secondary battery described above, when the resin layer melts and an internal short circuit is generated, there is a risk that the temperature rapidly increases locally at the short circuit point. In particular, in the case of a large battery for automobiles, or when an internal short circuit occurs at a location where heat is difficult to dissipate, the heat generated at that portion will rapidly increase the ambient temperature. As a result, there is a possibility that the activation energy of the thermal runaway reaction may be exceeded locally, and in that case, there is a possibility that the thermal runaway may eventually be caused.

  The present invention has been made to solve the problems of the conventional secondary battery described above. That is, the problem is to provide a lithium ion secondary battery that does not cause a thermal runaway reaction even when the temperature rises due to overcharging or the like.

  The lithium ion secondary battery of the present invention made for the purpose of solving this problem is a lithium ion secondary battery in which the first polarity electrode and the second polarity electrode are faced to each other while being insulated by an interelectrode separator, A conductive member facing the first polarity electrode, a resistance film provided on at least one surface of the first polarity electrode and the conductive member, and provided between the first polarity electrode and the conductive member to insulate them. In addition, a low melting point separator having a lower melting point than the interpolar separator is provided, and the second polar electrode and the conductive member are electrically connected.

  According to the lithium ion secondary battery of the present invention, the low melting point separator and the resistance film are provided between the conductive member connected to the second polarity electrode and the first polarity electrode. Since this low melting point separator has a lower melting point than the interelectrode separator, when the internal temperature of the lithium ion secondary battery rises due to overcharging or the like, the low melting point separator first melts. When the low melting point separator is melted, the first polarity electrode and the conductive member are conducted through the resistance film. Therefore, since the first polarity electrode and the second polarity electrode are conducted through the resistance film, a large current does not flow though a short-circuited state. That is, since the battery energy is slowly released, further increase in the internal temperature is suppressed. As a result, the lithium ion secondary battery does not cause a thermal runaway reaction even when the temperature rises due to overcharging or the like.

Furthermore, the present invention has a wound structure in which the first polarity electrode, the second polarity electrode, and the interelectrode separator are wound, and the first polarity electrode has a low melting point on both surfaces of the conductive member at the innermost periphery of the winding. It is desirable that they face each other via a separator, and resistance films are provided on both surfaces of the conductive member.
In this way, since the low melting point separator is provided in the innermost peripheral part where heat is most likely to accumulate, there is no possibility that the interelectrode separator will melt prior to the low melting point separator. Therefore, the lithium ion secondary battery has higher safety. Furthermore, since this resistance film is not provided between the first polarity electrode and the second polarity electrode, it does not hinder the power generation reaction. Therefore, there is no possibility that the battery performance is deteriorated by the resistance film.

Furthermore, in the present invention, a plurality of first polar electrodes and a plurality of second polar electrodes are laminated through interelectrode separators, and the first polar electrode is a low melting point separator on both surfaces of the conductive member. It is desirable that resistance films are provided on both surfaces of the conductive member.
In this way, the laminated lithium ion secondary battery is also a lithium ion secondary battery that does not cause a thermal runaway reaction even when the temperature rises due to overcharging or the like. Furthermore, since this resistance film is not provided between the first polarity electrode and the second polarity electrode, it does not hinder the power generation reaction. Therefore, there is no possibility that the battery performance is deteriorated by the resistance film.

  The lithium ion secondary battery of the present invention is a lithium ion secondary battery that does not cause a thermal runaway reaction even when the temperature rises due to overcharging or the like.

"First form"
Hereinafter, a first embodiment of the present invention will be described in detail with reference to the accompanying drawings. In this embodiment, the present invention is applied to a wound lithium ion secondary battery that occludes and releases lithium ions.

  The secondary battery 1 of this embodiment is a wound lithium ion secondary battery, as schematically shown in FIGS. FIG. 1 is a cross-sectional view in a direction perpendicular to the winding axis. FIG. 2 shows a wound body that is a main part of the secondary battery 1. As shown in FIG. 1, the secondary battery 1 is formed by winding a positive electrode plate 11 and a negative electrode plate 12 with an inter-electrode separator 13 interposed therebetween, and a short-circuit foil 14 and a low-power surrounding the short-circuit foil 14 at the winding center. A melting point separator 15 is provided. Furthermore, it has a positive electrode terminal 21 connected to the positive electrode plate 11, a negative electrode terminal 22 connected to the negative electrode plate 12, and a short-circuit terminal 24 connected to the short-circuit foil 14. As shown in FIG. 2, the positive electrode terminal 21 and the short-circuit terminal 24 are connected by welding or the like at the tip portion.

  The positive electrode plate 11 is formed in a strip shape by applying a positive electrode active material such as lithium cobalt oxide to an aluminum foil or the like. The negative electrode plate 12 is formed in a strip shape by applying a negative electrode active material such as a carbon material to a copper foil or the like. The inter-electrode separator 13 is an insulating band-shaped resin film or the like, and electrically separates the positive electrode plate 11 and the negative electrode plate 12. These are all the same as those used for general lithium ion secondary batteries. Note that no active material is applied to the positive terminal 21 and the negative terminal 22.

As shown in FIG. 1, the short-circuit foil 14 has a resistance film 35 provided on both surfaces of a conductive member 34 such as an aluminum foil. As the resistance film 35, for example, a film made of a metal oxide such as CuFeO ₂ , NiO, Cr ₂ O ₃ may be used. The short-circuit terminal 24 provided on the short-circuit foil 14 is formed of the same material as the conductive member 34 and is directly connected to the conductive member 34. The short-circuit terminal 24 is not provided with a resistance film 35. The low melting point separator 15 is an insulating resin film or the like, and has a lower melting point than the interelectrode separator 13. For example, a low molecular weight polyethylene film that melts at about 80 to 100 ° C. is preferable.

  Next, a method for manufacturing the secondary battery 1 will be described. First, the positive electrode plate 11, the negative electrode plate 12, and the short-circuit foil 14 are each manufactured. The positive electrode plate 11 and the negative electrode plate 12 are each manufactured by a general method, and a positive electrode terminal 21 and a negative electrode terminal 22 are connected in the vicinity of the end portions, respectively. These terminals are placed so that they do not overlap each other when wound.

  The short-circuit foil 14 is manufactured by dispersing fine particles such as metal oxide in water or an organic solvent, and forming a resistive film 35 on the conductive member 34 by dip coating or spin coating. Alternatively, the resistance film 35 may be produced by CVD. Further, a short-circuit terminal 24 is connected to the short-circuit foil 14. Alternatively, the short-circuit terminal 24 may be connected to the conductive member 34 in advance, and then the resistance film 35 may be formed. This short-circuit foil 14 is formed to be equal to or slightly smaller than the wound core. Moreover, the short circuit terminal 24 is good to arrange | position in the position which overlaps with the positive electrode terminal 21, when winding.

  Next, low melting point separators 15 are disposed on both sides of the short-circuit foil 14. The low melting point separator 15 is formed to be slightly larger than the short-circuit foil 14. Alternatively, one large low melting point separator 15 may be folded in half, and the short-circuit foil 14 may be sandwiched therebetween. Further, as shown in FIG. 3, the negative electrode plate 12 is overlaid on the short-circuit foil 14 sandwiched between the low melting point separators 15, and the interelectrode separator 13, the positive electrode plate 11, and the interelectrode separator 13 are overlaid in this order. These are wound by rotating as indicated by the broken line so that the short-circuit foil 14 is at the center. Thereby, the negative electrode plate 12 is disposed on the innermost periphery of the wound body, and the inner peripheral surface thereof faces the short-circuit foil 14 through the low melting point separator 15.

  Next, the end of the short-circuit terminal 24 is welded to the end of the positive terminal 21. Thereby, the wound body as shown in FIG. 2 is completed. Further, the wound body is sealed in a case, and an electrolytic solution is injected inside and sealed. Thereby, the wound type secondary battery 1 is manufactured.

  When the secondary battery 1 is overcharged, the internal temperature of the secondary battery 1 gradually increases due to an exothermic reaction. At this time, the temperature tends to be particularly high at the inner periphery where heat is difficult to escape. When the inner peripheral portion reaches the melting temperature of the low melting point separator 15, melting or deformation of the low melting point separator 15 disposed on the inner peripheral portion starts. Thereby, the short-circuit foil 14 and the negative electrode plate 12 are in partial contact.

  At this time, the temperature of the portion other than the inner peripheral portion is almost equal to or lower than the melting temperature of the low melting point separator 15. For this reason, there is no possibility that the interpolar separator 13 having a higher melting point than that of the low melting separator 15 is deformed or melted. Since the short-circuit foil 14 is provided with the resistance film 35 on both surfaces thereof, the current flowing through the contact portion with the negative electrode plate 12 flows through the resistance film 35. Therefore, this current does not become a large current but is limited. Therefore, the secondary battery 1 is not greatly increased in temperature by this short-circuit current, and there is no possibility of reaching the thermal runaway temperature.

  Next, the magnitude of the film resistance of the resistance film 35 formed on the short-circuit foil 14 will be described. In order to operate as described above, the film resistance R of the resistance film 35 needs to be set so as to be within a range between the lower limit resistance Ra and the upper limit resistance Rb described below. The lower limit resistance Ra of the film resistance R is determined by the heat generation limit of the short circuit portion. If the film resistance R is too small, the short-circuit current flowing in that portion is large and the amount of heat generation is large, which is inappropriate. Further, the upper limit resistance Rb of the film resistance R is determined by the charging current. If the film resistance R is too large, the charging current is larger than the current discharged from the short-circuited portion, so that overcharging does not stop and is inappropriate.

The lower limit resistance Ra is obtained as follows. First, A is the heat capacity of the positive electrode active material, ΔK is the difference between the cell temperature and the thermal runaway temperature during short circuit, V is the voltage during short circuit, S is the area of the short circuit part, and Ia is the current flowing through the short circuit part. The heat generation limit may be as long as the temperature rise when power (V · Ia) is added to the heat capacity A is smaller than ΔK. Therefore, it is expressed as
ΔK> (V · Ia) / A
Here, since Ia = V / (Ra / S),
ΔK> V ² / (A · Ra / S)
Therefore,
Ra> V ² · S / (ΔK · A) (Formula 1)

The upper limit resistance Rb is obtained as follows. First, the charging current is I, the current flowing through the short-circuited portion is Ib, the area of the short-circuited portion is S, and the voltage at the time of the short-circuit is V. The condition for stopping the overcharge is that the short-circuit current Ib is larger than the charge current I. Therefore, it is expressed as
Ib> I
Here, since Ib = V / (Rb / S),
I <V / (Rb / S)
Therefore,
Rb <(V · S) / I (Formula 2)

From Equation 1 and Equation 2, the range of film resistance R is expressed by the following equation.
V ² · S / (ΔK · A) <R <(V · S) / I (Formula 3)
The range of the film resistance R can be obtained by inputting each numerical value into Equation 3. The material and thickness of the film formed on the short-circuit foil 14 may be selected so that the film resistance within this range can be obtained.

For example, a charging current I = 10 A, a short-circuit area S = 1 cm ² , a short-circuit voltage V = 5 V, and a positive electrode active material heat capacity A = 0.8 J / K. Here, 100% of the Joule heat in the short-circuit portion is absorbed by 1 g of the positive electrode active material. If the thermal runaway temperature is 200 ° C. and the cell temperature at the time of short circuit is 50 ° C., the temperature difference ΔK = 150 ° C. By substituting these into Equation 3 above, the range of film resistance R required in this case is as follows.
0.2 <R <0.5 (Ω · cm ² )

FIG. 4 and FIG. 5 show the relationship between the film thickness, the resistance, and the result of the overcharge test when an aluminum foil coated with Cr ₂ O ₃ is used as the short-circuit foil 14. As the film thickness increases, the resistance value increases as shown in FIG. In the test with five film thicknesses shown in FIG. 4, the film resistance is 0.22, 0.33, 0.54 (Ω · cm ² ), and the temperature and voltage can be controlled without thermal runaway. It was able to fall and be in a safe state. From these experiments, the range of the film resistance R is considered to be almost appropriate.

Furthermore, the result of having performed the overcharge test about the secondary battery 1 of this form is shown in FIG. Here, the positive electrode active material of the positive electrode plate 11 is LiNiO ₂ , the negative electrode active material of the negative electrode plate 12 is graphite, the interelectrode separator 13 is polypropylene (PP), and the electrolyte is an organic solvent-based 10 Ah class cell (volume 300 ml). Was used. Further, an aluminum foil coated with Cr ₂ O ₃ to a thickness of 2.5 μm was used as the short-circuit foil 14, and polyethylene (PE) was used as the low melting point separator 15. As experimental conditions, CC charging was performed at a current of 10 A in an environment of a temperature of 25 ° C. Also in this figure, as in FIG. 9, the solid line shows the change in temperature, and the vertical axis is the left end side. A broken line indicates a change in voltage, and the vertical axis indicates the right end side.

  In the secondary battery 1, as shown in FIG. 6, the melting of the low melting point separator 15 started at time t, and a short circuit between the short-circuit foil 14 and the negative electrode plate 12 occurred. As a result, although the temperature rose temporarily, both voltage and temperature became almost constant later. As can be seen from the comparison with FIG. 9, it can be seen that the melting of the low melting point separator 15 starts at the time t earlier than the time t <b> 1 and at a temperature lower than that of the interelectrode separator 13. Furthermore, since the battery is slowly discharged through the short-circuit foil 14, the thermal runaway reaction does not occur even if the charging current continues to flow.

  As described in detail above, according to the secondary battery 1 of the present embodiment, since the short-circuit foil 14 and the low melting point separator 15 are provided, the low melting point separator 15 is first melted before reaching the thermal runaway temperature. Then, the short-circuit foil 14 and the negative electrode plate 12 are partially short-circuited, and the positive electrode plate 11 and the negative electrode plate 12 are short-circuited via the short-circuit foil 14. Since the resistance film 35 is formed on the short-circuit foil 14, the short-circuit current does not increase to some extent. Therefore, the lithium ion secondary battery does not cause a thermal runaway reaction even when the temperature rises due to overcharging or the like.

  Further, in the secondary battery 1 of this embodiment, the short-circuit foil 14 and the low melting point separator 15 are provided on the innermost peripheral portion of the secondary battery 1. The innermost peripheral portion is a position where heat is most likely to accumulate, and there is no possibility that the inter-electrode separator 13 in the other portion is melted before the low melting point separator 15 disposed at this position is melted. Further, since the arrangement of the short-circuit foil 14 is not between the positive electrode plate 11 and the negative electrode plate 12, it does not become an obstacle to the power generation reaction of the secondary battery 1. Therefore, the battery performance is not deteriorated by the short-circuit foil 14.

"Second form"
Hereinafter, a second embodiment of the present invention will be described in detail with reference to the accompanying drawings. In this embodiment, the present invention is applied to a stacked lithium ion secondary battery that occludes and releases lithium ions. Instead of winding as in the first embodiment, the secondary battery is formed by stacking layers. About the member which is common in the 1st form, the same numerals are attached and explanation is omitted.

  As shown in FIGS. 7 and 8, the secondary battery 2 of this embodiment is a stacked lithium ion secondary battery. FIG. 7 shows an electrode body which is a main part of the secondary battery 2, and FIG. 8 shows a large space between the layers in FIG. In the secondary battery 2, a plurality of positive plates 11 and negative plates 12 having a predetermined shape are alternately stacked with inter-electrode separators 13 interposed therebetween. Further, a short-circuit foil 14 is laminated near the center in the laminating direction. Low melting point separators 15 are disposed on both surfaces of the short-circuit foil 14, and the negative electrode plate 12 is disposed on the outer side thereof.

  Each positive electrode plate 11 is connected to a positive electrode terminal 21, and each negative electrode plate 12 is connected to a negative electrode terminal 22. As in the first embodiment, the short-circuit foil 14 has a resistance film 35 formed on a conductive member 34 and a short-circuit terminal 24 connected thereto. The positive electrode terminal 11 of each positive electrode plate 11 and the short-circuit terminal 24 of the short-circuit foil 14 are connected to each other by welding. The negative terminals 22 of the negative plates 12 are connected to each other by welding. Thereby, it becomes the secondary battery 2 which can be charged / discharged similarly to the secondary battery 1 of a 1st form.

  Also in the secondary battery 2 of this embodiment, when the overcharge state continues, the temperature gradually increases from the vicinity of the center. When the melting temperature of the low melting point separator 15 is reached, the low melting point separator 15 is deformed or melted, and the short-circuit foil 14 and the negative electrode plate 12 are partially short-circuited. Furthermore, the negative electrode plate 12 and the positive electrode plate 11 are in partial contact via the short-circuit foil 14. Here, since the resistance film 35 is formed on the short-circuit foil 14, the short-circuit current is not so large. Therefore, there is no risk of sudden temperature rise or partial temperature rise, and thermal runaway temperature is not reached.

  As described in detail above, the secondary battery 2 of this embodiment is a lithium ion secondary battery that does not cause a thermal runaway reaction even when the temperature rises due to overcharging or the like.

In addition, this form is only a mere illustration and does not limit this invention at all. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof.
For example, the short-circuit foil 14 is not necessarily limited to one place, and may be inserted into a plurality of places.
Further, for example, the arrangement and shape of each terminal are not limited to the above embodiments. A bipolar terminal may be disposed on both ends of the winding axis. Also in the wound secondary battery 1, the positive electrode terminal 21 and the negative electrode terminal 22 may be connected to a plurality of locations on the positive electrode plate 11 and the negative electrode plate 12.
Further, for example, the connection method between the short-circuit terminal 24 and the positive electrode terminal 21 of the short-circuit foil 14 is not limited to welding, and may be electrically connected.
For example, the short-circuit foil 14 may be connected to the negative electrode plate 12. In that case, a low melting point separator 15 is disposed between the short-circuit foil 14 and the positive electrode plate 11.
Further, for example, the material of each member described in each of the above embodiments is only an example and can be replaced with another material. The low melting point separator 15 and the interelectrode separator 13 may be formed of the same substance but different in molecular weight only.

It is sectional drawing which shows the secondary battery of a 1st form. It is a perspective view which shows the secondary battery of a 1st form. It is explanatory drawing which shows the method of manufacturing the secondary battery of a 1st form. It is explanatory drawing which shows the result of the overcharge test done by changing the film thickness of a short circuit foil. It is a graph which shows the relationship between the film thickness of a short circuit foil, and resistance. It is a graph which shows the result of the overcharge test by the rechargeable battery of the 1st form. It is a perspective view which shows the secondary battery of a 2nd form. It is explanatory drawing which shows the method of manufacturing the secondary battery of a 2nd form. It is a graph which shows the result of the overcharge test by the conventional secondary battery.

Explanation of symbols

1, 2 Secondary battery (Lithium ion secondary battery)
11 Positive plate (first polarity electrode, second polarity electrode)
12 Negative electrode (first polarity electrode, second polarity electrode)
13 Electrode separator 15 Low melting point separator 34 Conductive member 35 Resistance film

Claims (3)

In a lithium ion secondary battery in which a first polarity electrode and a second polarity electrode are faced to each other while being insulated by an interelectrode separator,
A conductive member facing the first polarity electrode;
A resistance film provided on at least one surface of the first polarity electrode and the conductive member;
A low melting point separator that is provided between the first polar electrode and the conductive member to insulate them and has a lower melting point than the interelectrode separator;
A lithium ion secondary battery, wherein the second polarity electrode is electrically connected to the conductive member. The lithium ion secondary battery according to claim 1,
A wound structure formed by winding the first polarity electrode, the second polarity electrode, and the inter-electrode separator;
The first polar electrode is opposed to both surfaces of the conductive member through the low melting point separator at the innermost periphery of the winding,
A lithium ion secondary battery, wherein resistance films are provided on both surfaces of the conductive member. The lithium ion secondary battery according to claim 1,
A laminated structure in which a plurality of first polar electrodes and a plurality of second polar electrodes are laminated via an inter-electrode separator;
The first polar electrode faces both surfaces of the conductive member via the low melting point separator;
A lithium ion secondary battery, wherein resistance films are provided on both surfaces of the conductive member.

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