Battery

Provided is a battery capable of providing both fixing durability of a protective film during normal use and peeling easiness of the protective film during operation of a safety valve. A lithium ion secondary battery (100) includes a protective film (140) fixed to a battery case (110) while covering a safety valve part (125) including a breakable portion (126). This protective film (140) has a first fixed portion (145) located around a valve-corresponding unfixed portion (144) and fixed to the battery case (110) and a second fixed portion (147) located more outside than the first fixed portion and fixed to the battery case (110) through an intermediate unfixed portion (146). They are configured such that the first fixed portion (145) first peels off when the safety valve part (125) operates, prompting peeling of the second fixed portion (147), thus releasing gas to the outside.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase application of International Application No. PCT/JP2009/062571, filed Jul. 10, 2009, and claims the priority of Japanese Application No. 2008-211872, filed Aug. 20, 2008, the contents of both of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a battery provided with a battery case having a non-restoring type safety valve part including a breakable portion and a protective film fixed to the battery case while covering the safety valve part to seal the safety valve part.

BACKGROUND ART

Heretofore, there has been known a battery provided with a battery case having a safety valve (an explosion-proof valve) of a non-restoring type and a protective film fixed to the battery case while externally covering the safety valve to seal the safety valve. This protective film is used to prevent foreign matters and liquid droplets such as water and oil from sticking to the safety valve, especially, its breakable portion having a thinner wall, thereby avoiding corrosion of the safety valve. For example, Patent Literatures 1 to 3 disclose a battery having such protective film.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, in the conventional battery, the protective film is fixed in annular and solid manner to the battery case around the safety valve without contacting a portion of the battery case facing the safety valve. Accordingly, if the surface area of the solidly fixed portion of the protective film is large, fixing strength between the protective film and the battery case becomes too strong. Even when the safety valve operates, that is, even when the safety valve is opened by increase in internal pressure, allowing gas to spout or blow out, the protective film is hard to separate or peel off from the battery case and hence the gas is not allowed to be released to the outside of the protective film.

On the other hand, if the surface area of the solidly fixed portion of the protective film is small, the protective film is easy to reliably peel off from the battery case when the safety valve operates, thereby surely enabling release of gas to the outside of the protective film. However, such protective film is also apt to peel off from the battery case during normal use, causing a problem with fixing durability of the protective film.

In the conventional battery in which the protective film is fixed to the battery case in solid manner (i.e., fixed around the safety valve without facing the safety valve), it is difficult to provide both the fixing durability of the protective film during normal use and the easiness of peeling of the protective film during operation of the safety valve.

The present invention has been made in view of the circumstances to solve the above problems and has a purpose to provide a battery capable of providing both the fixing durability of a protective film during normal use and the peeling easiness of the protective film during operation of the safety valve.

Solution to Problem

To achieve the above object, one aspect of the invention provides a battery comprising: a battery case provided with a non-restoring safety valve part that includes a breakable portion and will be opened when the breakable portion is broken; and a protective film fixed to the battery case while covering the safety valve part from outside to seal the safety valve part, wherein the protective film includes at least: a valve-corresponding unfixed portion located facing the breakable portion of the safety valve part and unfixed to the battery case; a first fixed portion located outside the valve-corresponding unfixed portion and fixed to the battery case; and a second fixed portion located outside the first fixed portion through an intermediate unfixed portion unfixed to the battery case, the second fixed portion being annular and fixed to the battery case.

In this battery, the protective film is fixed to the battery case through at least the first and second fixed portions and hence can have a large surface area of the entire fixed portions. Accordingly, the protective film is hard to separate or peel off from the battery case for long periods and fixing durability can be improved.

In this battery, on the other hand, when the safety valve part is opened by increase of internal pressure, allowing gas to spout or blow out, and the gas exerts pressure on the valve-corresponding unfixed portion (during operation of the safety valve), at least a part of the first fixed portion peels off from the battery case earlier than the second fixed portion. Then, the pressure receiving area of the protective film which receives gas pressure additionally includes a peeled part of the first fixed portion. The pressure receiving area is therefore increased and large force acts on the second fixed portion, making the second fixed portion easy to peel off from the battery case. Peeling of the second fixed portion is thus prompted. Then, the inside and the outside of the second fixed portion are communicated with each other, releasing the gas to the outside. In this way, the peeling easiness of the protective film during operation of the safety valve can be improved.

Consequently, the present battery can provide both the fixing durability of the protective film during normal use and the peeling easiness of the protective film during operation of the safety valve.

As examples of the configuration of the “safety valve part”, there are a configuration integrally formed with the battery case, a configuration made in a separate component from the battery case and then bonded to the battery case, and so on.

The shape of the “battery case” may be for example cylindrical, rectangular parallelepiped, etc.

The shape of the “protective film” (in plan view) may be for example rectangular, circular, oblong, elliptic, polygonal, etc.

As examples of the configuration of the “first fixed portion”, there are an annular form surrounding the valve-corresponding unfixed portion or an intermittent form intermittently located around the valve-corresponding unfixed portion. The “first fixed portion” and the “second fixed portion” may be configured to be completely separated by the intermediate unfixed portion or be partially continuous.

The fixing configuration of the “first fixed portion” and the “battery case” may be achieved by for example fixing by adhesion, fixing by solder, fixing by welding, or the like. The same applies to the fixing configuration of the “second fixed portion” and the “battery case”. Furthermore, the fixing configuration of the “first fixed portion” and the “battery case” and that of the “second fixed portion” and the “battery case” may be similarly made by adhesion or achieved by different ways; one by adhesion and the other by solder.

Furthermore, in the above battery, preferably, the first and second fixed portions are configured such that, when the safety valve part is opened by increase in internal pressure, allowing gas to spout out, and gas pressure of the gas acts on the valve-corresponding unfixed portion, at least a part of the first fixed portion peels off from the battery case earlier than the second fixed portion, a pressure receiving area of the protective film which receives the gas pressure is increased, prompting the second fixed portion to peel off, and at least a part of the second fixed portion peels off from the battery case to release the gas to outside.

Accordingly, both the fixing durability of the protective film during normal use and the peeling easiness of the protective film during operation of the safety valve can be achieved.

Furthermore, in the above battery, preferably, the first fixed portion has an annular shape surrounding the valve-corresponding unfixed portion.

In this battery, the first fixed portion is annular to surround the valve-corresponding unfixed portion. During normal use, therefore, the safety valve part is shielded from the case outside by not only the second fixed portion but also first fixed portion. This makes it possible to prevent foreign matters and liquid droplets such as water and oil from sticking to the safety valve part, especially its breakable portion. Accordingly, the effect of preventing corrosion of the safety valve part can be improved more effectively.

Since the first fixed portion is annular as above, the gas spouting from the safety valve during operation of the safety valve first presses only the valve-corresponding portion (not presses the intermediate fixed portion), thereby prompting the first fixed portion to peel off. When at least a part of the first fixed portion peels off, providing communication between the inside and the outside of the first fixed portion, the gas also reaches the intermediate unfixed portion located outside the first fixed portion. Accordingly, the pressure receiving area of the protective film which receives gas pressure additionally includes a peeled part of the first fixed portion and also the intermediate unfixed portion, so that the pressure receiving area is increased at once. Large force then acts on the second fixed portion, making it easy to peel off from the battery case. Peeling of the second portion is thus prompted. The inside and the outside of the second fixed portion are then communicated with each other, releasing gas to the outside. Consequently, the peeling easiness of the protective film during operation of the safety valve can be improved.

Moreover, in one of the above batteries, preferably, the first and second fixed portions are separated by the intermediate unfixed portion.

In this battery, the first and second fixed portions are separated by the intermediate unfixed portion. Accordingly, stepwise peeling can be reliably caused during the operation of the safety valve. Especially in the case where the first fixed portion has an annular shape surrounding the valve-corresponding unfixed portion, when at least a part of the first fixed portion peels off during operation of the safety valve, providing communication between the inside and the outside of the first fixed portion, gas reaches the entire intermediate unfixed portion located outside the first fixed portion. The pressure receiving area of the protective film which receives gas pressure is particularly increased. This reliably induces the second fixed portion to peel off. Consequently, the peeling easiness of the protective film during operation of the safety valve can be improved especially.

In one of the above batteries, further preferably, the battery case includes: a protrusion being located around the breakable portion and configured to protrude outside and have a top surface; and a low-lying portion being located around the protrusion and having a low-lying surface located lower than the top surface, the first fixed portion is fixed to the top surface of the protrusion, and the second fixed portion is fixed to the low-lying surface of the low-lying portion.

In this battery, the protrusion is provided in the battery case and the first fixed portion is fixed to the top surface of this protrusion, and the low-lying portion is provided in the battery case and the second fixed portion is fixed to the low-lying surface of the low-lying portion. Accordingly, the intermediate unfixed portion located between the first and second fixed portions can be made slant. An actual area of the intermediate unfixed portion can be made larger than the area of the intermediate unfixed portion when the protective film fixed to the battery case is viewed in plan view. Specifically, the actual area of the intermediate unfixed portion can be increased without the need of increasing the outer dimension of the protective film fixed to the battery case. Accordingly, the pressure receiving area of the protective film during operation of the safety valve can be increased by just that much, so that the second fixed portion reliably peels off from the battery case. Consequently, the peeling easiness of the protective film during operation of the safety valve can be improved.

In one of the above batteries, further preferably, the first and second fixed portions are respectively fixed to the battery case by adhesion.

Since the first and second fixed portions are fixed to the battery case by adhesion, the protective film can be easily and reliably fixed to the battery case. Accordingly, the battery can be provided at low cost and the fixing durability of the protective film can be sufficiently ensured.

Concrete adhering configuration includes for example a configuration that an adhesive layer is provided on the protective film and this protective film is bonded to the battery case, a configuration that an adhesive layer is provided on the battery case and the protective film is bonded to the battery case, and others.

In the above battery, preferably, the protective film includes a main body and an adhesive layer formed on an entire surface of the main body on a side to be fixed to the battery case.

In the case where the protective film is fixed to the battery case by adhesion, only a fixing portion between the protective film and the battery case, that is, the first and second fixed portions and others have only to be adhered. However, it costs to partially form an adhesive layer on the protective film main body and position and bond it to the battery case. It is therefore difficult to provide the battery at low cost. In the present battery, on the other hand, the adhesive layer is formed on the entire area of the protective film main body (the entire surface of the side to be fixed to the battery case), so that the protective film can be provided at low cost, resulting in an inexpensive battery. Moreover, it is easy to position the protective film with respect to the battery case in bonding operation.

Moreover, in one of the above batteries, preferably, the protective film is made of resin.

Since the protective film is made of resin, the protective film can be provided at lower cost than in the case where the protective film is made of metal foil or the like. This battery can also be provided at low cost. The protective film made of resin can easily have flexibility. Accordingly, even when the battery case has protrusions and recesses, the protective film can be easily and reliably fixed to the battery case and hence the fixing durability of the protective film can be sufficiently ensured. Particularly, the protective film may be transparent to allow easy observation of a state of the safety valve (the breakable portion).

DESCRIPTION OF EMBODIMENTS

First Embodiment

A detailed description of a preferred first embodiment of the present invention will now be given referring to the accompanying drawings.FIG. 1schematically shows a lithium ion secondary battery (a sealed battery)100in the first embodiment.FIGS. 2 to 4show a safety valve part125and its surrounding in the lithium ion secondary battery100.FIG. 2shows a state including a protective film140andFIG. 3shows a state including no protective film140.FIG. 4is a sectional view taken along a line A-A inFIG. 2. This lithium ion secondary battery100is a rectangular battery to be mounted in a hybrid electric car or an electric car and used for a driving source.

This lithium ion secondary battery100includes a battery case110, an electrode body not shown and housed in the battery case110, a positive terminal161and a negative terminal163fixedly provided on the battery case110, and others (seeFIG. 1). The lithium ion secondary battery100further includes the protective film140fixed or stuck to the battery case110while covering the safety valve part125provided in the battery case110to protect the safety valve part125from outside.

The battery case110is made of metal in a rectangular parallelepiped shape. The battery case110includes a case upper wall111, a case lower wall112facing the upper wall111, and four case side walls113,114,115, and116each joining the upper and lower walls111and112. Each of the upper wall111, the lower wall112, the side walls113,114,115, and116has an almost flat plate shape.

Of them, the case upper wall111will be described in detail below. The case upper wall111has a narrow rectangular shape in plan view and is welded, at its peripheral edge portion, to the case side walls113,114,115, and116respectively. At predetermined locations near both ends of the case upper wall111in a longitudinal direction, the positive terminal161and the negative terminal163are fixedly provided (seeFIG. 1). The positive terminal161is electrically connected to a positive current collector of the electrode body not shown inside the battery while protruding upward from the case upper wall111for the use of electric connection to the outside. The negative terminal163is electrically connected to a negative current collector of the electrode body not shown inside the battery while protruding upward from the case upper wall111for the use of electric connection to the outside.

At a predetermined location nearer to the negative terminal163than the center of the case upper wall111, a liquid inlet121is formed for pouring of electrolyte into the battery case110.

Furthermore, in the center of the case upper wall111, a case opening123having a rectangular shape in plan view is provided. Under this case opening123(inside the battery), the safety valve part125having a rectangular shape in plan view is provided so as to close the case opening123(seeFIG. 4). This safety valve part125is integrally formed with the case upper wall111to constitute a part of the case upper wall111. The safety valve part125is formed as a thin film thinner than the case upper wall111and includes on the upper side with a breakable portion126formed as a V-shaped groove. Accordingly, the safety valve part125is caused to operate when the internal pressure of the battery reaches a predetermined pressure. In other words, the breakable portion126is broken or cleaved when the internal pressure reaches the predetermined pressure, thus releasing internal gas to the outside (seeFIGS. 6 and 7).

Around the breakable portion126and further around the case opening123of the case upper wall111, an inside annular protrusion (protrusion)128is formed integral with the case upper wall111to have a rectangular annular shape surrounding the breakable portion126and the case opening123and protruding upward (seeFIGS. 2 to 4). This inside annular protrusion128includes a flat top surface128nin a rectangular shape in plan view.

Around the inside annular protrusion128of the case upper wall111, an annular low-lying portion (a low-lying portion)131is formed to have a rectangular annular shape surrounding the inside annular protrusion128and be placed lower than the inside annular protrusion128. An upper surface of this annular low-lying portion131is a flat low-lying surface131nhaving a rectangular shape in plan view and located in a lower place than a top surface128nof the inside annular protrusion128.

Around the annular low-lying portion131of the case upper wall111, a circumferential annular protrusion133is integrally formed with the case upper wall111to have a rectangular annular shape surrounding the annular low-lying portion131and protruding upward. This protrusion133constitutes a peripheral edge portion of the case upper wall111. This protrusion133has a top surface133nlocated higher (above) the low-lying surface131nof the annular low-lying portion131and higher (above) the top surface128nof the inside annular protrusion128. This top surface133nis a flat surface rectangular in plan view.

On the case upper wall111, a protective film140is peelably fixed while covering the case opening123(the safety valve part125) from outside of the case (from above). Specifically, the protective film140is stuck to the case upper wall111by adhesion.

This protective film140includes a main body141as a base material, and an adhesive layer142formed over a lower surface141bof the protective film141on the side to be fixed to the case upper wall111. This protective film140is made of resin. To be concrete, the main body141is made of synthetic resin (PPS in this embodiment) and the adhesive layer142is made of acrylic adhesive.

This protective film140includes a valve-corresponding unfixed portion144, a first fixed portion145, an intermediate unfixed portion146, and a second fixed portion147.

The valve-corresponding unfixed portion144is a rectangular portion in plan view placed over the case opening123to face the breakable portion126and hence the safety valve part125. The unfixed portion144is unfixed to the case upper wall111.

The first fixed portion145is located outside the valve-corresponding unfixed portion144(on the peripheral edge side than the valve-corresponding unfixed portion144) and stuck to the case upper wall111around the case opening123. Specifically, the first fixed portion145is a rectangular annular shape surrounding the case opening123and stuck to the top surface128nof the inside annular protrusion128of the case upper wall111by the adhesive layer142.

The second fixed portion147is located outside the first fixed portion145via the intermediate unfixed portion146mentioned below and stuck to the case upper wall111. To be more specific, the second fixed portion147has a rectangular annular shape surrounding the first fixed portion145through the intermediate unfixed portion146and stuck to the low-lying surface131nof the annular low-lying portion131of the case upper wall111by the adhesive layer142.

The intermediate unfixed portion146is located between the first fixed portion145and the second fixed portion147and unfixed to the case upper wall111. This intermediate unfixed portion146is rectangular annular to surround the first fixed portion145to completely separate the first fixed portion145and the second fixed portion147. As described above, the first fixed portion145is stuck to the top surface128nof the inside annular protrusion128and the second fixed portion147is stuck to the low-lying surface131nof the annular low-lying portion131. Accordingly, the intermediate unfixed portion146is made slant with respect to the top surface128nand the low-lying surface131n. A clearance (space) CK1is formed between the intermediate unfixed portion146and the case upper wall111.

In this lithium ion secondary battery100, the protective film140is fixed to the battery case110(in particular, the case upper wall111) through two fixed portions, i.e., the first fixed portion145and the second fixed portion147and hence can have a large surface area of the entire fixed portions. The protective film140is therefore hard to peel off from the battery case110for long periods and can have improved fixing durability.

In this lithium ion secondary battery100, on the other hand, the safety valve part125operates when the internal pressure of the battery reaches the predetermined pressure (seeFIG. 5). That is, the breakable portion126of the safety valve part125is broken or cleaved, allowing gas to spout out through the safety valve part125. The gas pressure of this spouting gas firstly acts on only the valve-corresponding unfixed portion144of the protective film140facing the safety valve part125(the breakable portion126).

A resultant force causes at least a part of the first fixed portion145of the two fixed portions (the first fixed portion145and the second fixed portion147) to peel off from the battery case110(concretely, from the top surface128nof the inside annular protrusion128of the case upper wall111) earlier than the second fixed portion147(seeFIG. 6).

When at least a part of the first fixed portion145peels off, allowing communication between the inside and the outside of the first fixed portion145, a gas pressure-receiving area is increased at once. Specifically, the spouting gas also reaches the entire intermediate unfixed portion146located outside the first fixed portion145. Thus, the pressure-receiving area of the protective film140which receives gas pressure additionally includes a peeled part of the first fixed portion145and also the entire intermediate unfixed portion146. Consequently, the pressure-receiving area is increased at once.

A resultant large force then acts on the second fixed portion147, making it easy to peel off from the battery case110. Peeling of the second fixed portion147thus advances. As shown inFIG. 7, the second fixed portion147is also peeled off from the battery case110(i.e., the low-lying surface131nof the annular low-lying portion131of the case upper wall111), thereby allowing communication between the inside and the outside of the second fixed portion147, releasing the gas to the outside. Such stepwise peeling makes it possible to enhance the peeling easiness of the protective film140during operation of the safety valve part125.

The lithium ion secondary battery100in the first embodiment can consequently provide both the fixing durability of the protective film140during normal use and the peeling easiness of the protective film140during operation of the safety valve.

In the lithium ion secondary battery100, furthermore, the first fixed portion145and the second fixed portion147are formed annularly as described above. Accordingly, the safety valve part125during normal use is reliably shielded from the outside of the case by those fixed portions145and147(seeFIG. 4). This makes it possible to prevent foreign matters and liquid droplets from sticking to the safety valve part125, especially, the breakable portion126thereof, thereby further improving the effect of avoiding corrosion of the safety valve part125.

In the lithium ion secondary battery100, furthermore, the first fixed portion145and the second fixed portion147are separated by the intermediate unfixed portion146, so that the aforementioned stepwise peeling can be reliably conducted during operation of the safety valve. In other words, the first fixed portion145is made annular as described above. When the first fixed portion145peels off during operation of the safety valve, thereby allowing communication between the inside and the outside of the first fixed portion145, the gas reaches the entire intermediate unfixed portion146located outside the first fixed portion145. The pressure-receiving area is remarkably increased. The second fixed portion147therefore reliably peels off, particularly improving the peeling easiness of the protective film140during operation of the safety valve.

In the lithium ion secondary battery100, furthermore, the intermediate unfixed portion146located between the first and second fixed portions145and147is made slant as described above (seeFIG. 4). Therefore, an actual area of the intermediate unfixed portion146is larger than the area of the intermediate unfixed portion146of the protective film140fixed to the battery case110and seen in plan view (seeFIG. 2). In other words, the actual area of the intermediate unfixed portion146is made large without increasing the outer dimension of the protective film140stuck to the battery case110. When the area of the intermediate unfixed portion146is large, the protective film140comes to have a wide pressure-receiving area by just that much after the first fixed portion145peels off from the battery case110. Thus, the second fixed portion147can be reliably peeled off from the battery case110. This makes it possible to further improve the peeling easiness of the protective film140during operation of the safety valve.

In the lithium ion secondary battery100, furthermore, the first fixed portion145and the second fixed portion147are fixed to the battery case110by adhesion, so that the protective film140can be easily and reliably fixed to the battery case110. Accordingly, the lithium ion secondary battery100can be provided at low cost and the fixing durability of the protective film140can be sufficiently ensured.

In the lithium ion secondary battery100in the first embodiment, the adhesive layer142is formed over the entire lower surface141bof the main body141. The protective film140can therefore be provided at lower cost as compared with an adhesive layer formed on only a part of the lower surface141b. Consequently, the lithium ion secondary battery100can also be provided at low cost in this view. Furthermore, as compared with the adhesive formed on only a part of the protective film, it is easier to position the protective film140with respect to the battery case110for sticking.

In the lithium ion secondary battery100, furthermore, the protective film140is made of resin, so that the protective film140can be provided at lower cost than in the case where the protective film140is made of metal foil or the like. The lithium ion secondary battery100can therefore be provided at lower cost. Since the protective film140is made of resin, the protective film140can easily have flexibility. Even when there are protrusions and recesses like the case upper wall111in the first embodiment, the protective film140can be easily and reliably fixed to the battery case110. The fixing durability of the protective film140can thus be sufficiently ensured.

Second Embodiment

A second embodiment will be described below.FIGS. 8 and 9show a safety valve part225and its surrounding in a lithium ion secondary battery200in the second embodiment. The lithium ion secondary battery200in the second embodiment is substantially identical to the lithium ion secondary battery100in the above first embodiment excepting that a case upper wall211has no inside annular protrusion and the case upper wall211is formed with an annular groove229. Accordingly, the explanations of the similar or identical components to those in the first embodiment are omitted or simplified.

The lithium ion secondary battery200in the second embodiment includes, as with the first embodiment, a battery case210, an electrode body not shown, a positive terminal161and a negative terminal163, a protective film240, and others (seeFIG. 1).

Of the battery case210, a case lower wall112and case side walls113,114,115, and116are identical to those of the battery case110in the first embodiment. The form of the case upper wall211is different from the case upper wall111in the first embodiment.

This case upper wall211is formed with a case opening223having a rectangular shape in plan view as with the first embodiment. However, no annular protrusion is present around the case opening223. An inside annular portion228having a rectangular annular shape is formed surrounding the case opening223. An upper surface of this inside annular portion228is a flat annular surface228nhaving a rectangular annular shape in plan view.

Around the inside annular portion228of the case upper wall211, an annular recessed groove229is formed having a rectangular annular shape surrounding the inside annular portion228and recessed downward than the annular surface228n.

Around the annular groove229of the case upper wall211, an outside annular portion231is provided having a rectangular annular shape surrounding the annular groove229. An upper surface of this outside annular portion231is an annular surface231nthat is a flat rectangular surface in plan view and is located at the same height as the annular surface228nof the inside annular portion228.

Around the outside annular portion231of the case upper wall211, a circumferential annular protrusion233is integrally formed with the case upper wall211to have a rectangular annular shape surrounding the outside annular portion231and protruding upward. This circumferential annular protrusion233, similar to the circumferential annular protrusion133in the first embodiment, has a top surface233nto constitute the peripheral edge portion of the case upper wall211.

On the case upper wall211, a protective film240is peelably fixed (concretely, is adhered) while covering the case opening223(the safety valve part225) from outside of the case. This protective film240includes a main body241and an adhesive layer242formed entirely on a lower surface241bof the main body241as with the protective film140in the first embodiment.

This protective film240includes a valve-corresponding unfixed portion244, a first fixed portion245, an intermediate unfixed portion246, and a second fixed portion247.

The valve-corresponding unfixed portion244is placed over the case opening223to face the safety valve part225and has a rectangular shape in plan view and is unfixed to the case upper wall211.

The first fixed portion245is fixed to the case upper wall211around the breakable portion226and the case opening223. Specifically, the first fixed portion245has a rectangular annular shape surrounding the case opening223and is stuck to the annular surface228nof the inside annular portion228of the case upper wall211by the adhesive layer242.

The second fixed portion247is located outside than the first fixed portion245and fixed to the case upper wall211. Specifically, the second fixed portion247has a rectangular annular shape surrounding the first fixed portion245through the intermediate unfixed portion246described below and is stuck to the annular surface231nof the outside annular portion231of the case upper wall211by the adhesive layer242. The intermediate unfixed portion246is located between the first and second fixed portions245and247and over the annular groove229. The intermediate unfixed portion246is not fixed to the case upper wall211. A clearance (space) CK2is thus formed between the intermediate unfixed portion246and the case upper wall211(the annular groove229). This intermediate unfixed portion246is rectangular annular to surround the first fixed portion245, thus completely separating the first fixed portion245and the second fixed portion247.

In this lithium ion secondary battery200, the protective film240is fixed to the battery case210(concretely, the case upper wall211) through the two fixed portions, i.e., the first and second fixed portions245and247and hence can have a large surface area of the entire fixed portions. Thus, the protective film240is hard to peel off from the battery case210for long periods and hence can have improved fixing durability.

On the other hand, in this lithium ion secondary battery200, the safety valve part225operates when the internal pressure of the battery reaches the predetermined pressure. Specifically, the safety valve part225is broken at the breakable portion226, causing gas to spout out through the safety valve part225. Gas pressure of this spouting gas first acts on only the valve-corresponding unfixed portion244of the protective film240facing the safety valve part225(the breakable portion226).

This pressure causes at least a part of the first fixed portion245of the two fixed portions (the first and second fixed portions245and247) to peel off from the battery case210earlier than the second fixed portion247. When the first fixed portion245peels off, providing communication between the inside and the outside of the first fixed portion245, the gas pressure receiving area is increased at once. In other words, the spouting gas also reaches the entire intermediate unfixed portion246located outside the first fixed portion245. Accordingly, the pressure receiving area of the protective film240which receives gas pressure additionally includes a peeled part of the first fixed portion245and also the entire intermediate unfixed portion246. Consequently, the pressure receiving area is increased at once.

A resultant large force then acts on the second fixed portion247, prompting peeling of the second fixed portion247from the battery case210. This second fixed portion247also peels off from the battery case210, thereby allowing communication between the inside and the outside of the second fixed portion247, releasing the gas to the outside. Such stepwise peeling makes it possible to improve the peeling easiness of the protective film240during operation of the safety valve part225.

The lithium ion secondary battery200in the second embodiment can also provide both the fixing durability of the protective film240during normal use and the peeling easiness of the protective film240during operation of the safety valve. In addition, other parts identical to those in the first embodiment can provide the same operations and advantages as in the first embodiment.

Third Embodiment

A third embodiment will be described below.FIGS. 10 and 11show a safety valve part325and its surrounding in a lithium ion secondary battery300in the third embodiment. The lithium ion secondary battery300in the third embodiment is different from the lithium ion secondary batteries100and200in the first and second embodiments in that a case upper wall311has no annular protrusion and an annular groove around a case opening323. Furthermore, the lithium ion secondary battery300is different from the lithium ion secondary batteries100and200in the first and second embodiments in that an adhesive layer342of a protective film340is partially, not entirely, formed on a protective film main body341. Others are substantially identical to those in the first or second embodiment. Identical parts to those in the first or second embodiment are explained briefly or not explained.

The lithium ion secondary battery300in the third embodiment, as with the first embodiment, includes a battery case310, an electrode body not shown, a positive terminal161and a negative terminal163, a protective film340, and others (seeFIG. 1).

Of the battery case310, a case lower wall112and case side walls113,114,115, and116are identical to those of the battery case110in the first embodiment. The form of the case upper wall311is different from the case upper walls111and112in the first and second embodiments.

This case upper wall311is formed with a case opening323having a rectangular shape in plan view as with the first embodiment. However, neither annular protrusion nor annular groove is present around the case opening323. Around the case opening323, a rectangular annular portion331is formed to annularly surround the case opening323. An upper surface of this annular portion331is a flat annular surface331nhaving a rectangular annular shape in plan view.

Around the annular portion331of the case upper wall311, a circumferential annular protrusion333having a rectangular annular shape surrounding the annular portion331and protruding upward is integrally formed with the case upper wall311. This protrusion333, as with the circumferential annular protrusion133in the first embodiment, includes a top surface333nand forms the peripheral edge portion of the case upper wall311.

The protective film340is fixed (concretely, is adhered) to the case upper wall311while covering the case opening323(the safety valve part325) from outside of the case. This protective film340includes a main body341and an adhesive layer342partially formed on a lower surface341bof the main body341on the side to be fixed to the case upper wall311. This adhesive layer342includes an inside adhesive portion342pin a rectangular annular shape located in a predetermined inside place and an outside adhesive portion342qin a rectangular annular shape located in a predetermined place more outside than the inside adhesive portion342p.

In the protective film340, a portion with the inside adhesive portion342pis a first fixed portion345and a portion with the outside adhesive portion342qis a second fixed portion347. A central portion located inside of the first fixed portion345and provided with no adhesive layer342is a valve-corresponding unfixed portion344. A portion located between the first and second fixed portions345and347and provided with no adhesive layer342is an intermediate unfixed portion346.

The valve-corresponding unfixed portion344is a rectangular portion in plan view facing the safety valve part325and located over the case opening323but not fixed to the case upper wall311.

The first fixed portion345is fixed to the case upper wall311around the breakable portion326and the case opening323. To be more concrete, the first fixed portion345has a rectangular annular shape surrounding the case opening323and stuck to the annular surface331nof the annular portion331of the case upper wall311by the adhesive layer342(i.e., the inside adhesive portion342p).

The second fixed portion347is located outside the first fixed portion345and fixed to the case upper wall311. To be concrete, the second fixed portion347has a rectangular annular shape surrounding the first fixed portion345through the intermediate unfixed portion346and stuck to the annular surface331nof the annular portion331of the case upper wall311by the adhesive layer342(i.e., the outside adhesive portion342q).

The intermediate unfixed portion346is located between the first and second fixed portions345and347but not fixed to the case upper wall311. This intermediate unfixed portion346has a rectangular annular shape surrounding the first fixed portion345to completely separate the first and second fixed portions345and347from each other.

In this lithium ion secondary battery300, the protective film340is fixed to the battery case310(concretely, the case upper wall311) through the two fixed portions, i.e., the first and second fixed portions345and347and hence can have a large surface area of the entire fixed portions. Accordingly, the protective film340is hard to peel off from the battery case310for long periods and can have improved fixing durability.

In this lithium ion secondary battery300, on the other hand, the safety valve part325operates when the internal pressure in the battery reaches the predetermined pressure. In other words, the safety valve part325is broken at the breakable portion326, allowing gas to spout out through the safety valve part325. Gas pressure of the spouting gas first acts on only the valve-corresponding unfixed portion344of the protective film340facing the safety valve part325(the breakable portion326).

This pressure causes at least a part of the first fixed portion345of the two fixed portions (the first and second fixed portions345and347) to peel off from the battery case310earlier than the second fixed portion347. When the first fixed portion345peels off, providing communication between the inside and the outside of the first fixed portion345, the pressure receiving area of the gas is increased at once. In other words, the spouting gas also reaches the entire intermediate unfixed portion346located outside the first fixed portion345. Accordingly, the pressure receiving area of the protective film340which receives gas pressure additionally includes a peeled part of the first fixed portion345and also the entire intermediate unfixed portion346. The pressure receiving area is increased at once.

A resultant large force acts on the second fixed portion347, prompting peeling of the second fixed portion347from the battery case310. This second fixed portion347also peels off from the battery case310and the inside and the outside of the second fixed portion347are communicated with each other, releasing the gas to the outside. Such stepwise peeling makes it possible to improve the peeling easiness of the protective film340during operation of the safety valve part325.

The lithium ion secondary battery300in the third embodiment can also provide both the fixing durability of the protective film340during normal use and the peeling easiness of the protective film340during operation of the safety valve. In addition, other parts identical to those in the first embodiment can provide the same operations and advantages as in the first embodiment.

The present invention is explained along the above embodiments but not limited to the first to third embodiments. The present invention may be embodied in other specific forms without departing from the essential characteristics thereof.

For instance, the first to third embodiments exemplify the safety valve parts125,225, and325integrally formed in the case upper walls111,211, and311respectively. Alternatively, the present invention may be applied to a configuration that the safety valve part is made as a separate component from the battery case and then is bonded to the battery case.

The first to third embodiments exemplify the safety valve parts125,225, and325provided on the lower side (inside the case) of the case openings123,223, and323respectively. Alternatively, the safety valve parts125,225, and325may be provided on the upper side (outside the case) of the case openings123,223, and323respectively.

Furthermore, the first to third embodiments exemplify the first fixed portions145,245, and345of the protective films140,240, and340, formed in an annular shape surrounding the case openings123,223, and323, respectively. Alternatively, the first fixed portions145,245, and345may be formed in a dotted island shape intermittently located around the case openings123,223, and323respectively.

For instance,FIG. 12shows a lithium ion secondary battery400in a modified example of the third embodiment, in which first fixed portions445of a protective film440are intermittently present like dotted islands around the breakable portion326and the case opening325.

The first to third embodiments furthermore exemplify that the first fixed portion145,245,345and the second fixed portion147,247,347are completely separated by the intermediate unfixed portion146,246,346. Alternatively, the first fixed portion145,245,345and the second fixed portion147,247,347may be partially continuous.

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