Patent Publication Number: US-2003232236-A1

Title: Battery package vent

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates generally to a method and apparatus for venting or expelling gas generated or evolved within an electrochemical cell container, and more particularly to a method and apparatus employing heat-sensitive materials for venting or expelling pressurized gas generated or evolved within an electrochemical cell container.  
       BACKGROUND OF THE INVENTION  
       [0002] Electrochemical cells or batteries typically consist of a cathode, an anode, and a liquid electrolyte or other material interposed there between for facilitating movement of ions between the anode and cathode. Batteries are commonly classified as either “primary” (non-rechargeable), or “secondary” (rechargeable).  
       [0003] Lithium-ion polymer batteries are a type of secondary battery. Most conventional lithium-ion polymer batteries use a relatively soft or flexible packaging or container, and employ a solid polymer electrolyte. As a result, lithium-ion polymer batteries are relatively thin (e.g., less than 10 mm thick), lightweight and easily shaped into different configurations and shapes.  
       [0004] A lithium-ion polymer battery typically includes one or more electrochemical cells, and employs the protective packaging or container portion to protect the electrochemical cell or cells from air and/or moisture infiltration. The packaging may be a flexible bag, pouch, or other similar package  
       [0005] Lithium-ion polymer cells evolve gas during storage as well as during and following service use. In some cases, a small volume of gas is typically evolved during normal use and storage, and at ambient temperatures. In many cases, an excessive volume of gas is generated or evolved, and therefore pressure is built-up, at elevated or abnormal temperatures. This gas may form for any number of reasons, e.g. breakdown of the electrolyte. When the battery packaging is sealed closed, excessive build-up of gas within the package must be vented or expelled to avoid any harm to the structural integrity and/or damage to the container, the electrochemical cells and/or the device in which the battery is employed.  
       [0006] Conventional venting systems typically employ pressure sensitive means for releasing gas evolved or generated within a container. However, such systems are inadequate because they require that excess amounts of pressure build before the vent system will release the gas, and thus may affect the structural integrity and/or may cause damage to the container over time. In contrast, the present invention employs temperature-sensitive means for releasing gas evolved or generated within a container, wherein the temperature sensitive means allows gas generated or evolved in the container to be vented or expelled, even if in small amounts. Thus, the present invention preemptively vents or expels gas before the gas builds to pressure levels sufficient to affect the structural integrity of the container.  
       [0007] Therefore, there exists a need for a method and apparatus for venting or expelling gas generated or evolved inside a battery package based on the operating temperature of the battery.  
       SUMMARY OF THE INVENTION  
       [0008] In accordance with one embodiment of the present invention, a battery container for enclosing an electrochemical cell is provided, wherein gas is generated or evolved within the container when the operational temperature of the battery is at or above a given minimum temperature. The container includes a sealable vent and a sealant in communication therewith for selectively and substantially sealing the vent, wherein the sealant is a heat sensitive material that substantially seals the vent, but softens or melts when the electrochemical cell is operating at or above the minimum temperature, thus unsealing the vent so as to allow the gas to escape from the container through the vent.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009] The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
     [0010]FIG. 1 is a perspective view of a conventional battery, in accordance with the prior art;  
     [0011]FIG. 2 is a sectional view taken along line  2 - 2  of FIG. 1, in accordance with the prior art;  
     [0012]FIG. 3 is an exploded view of a battery system, in accordance with one embodiment of the present invention;  
     [0013]FIG. 4 is a perspective view of the assembled battery system depicted in FIG. 4, in accordance with one embodiment of the present invention;  
     [0014]FIG. 5 is a sectional view taken along line  5 - 5  of FIG. 4, in accordance with one embodiment of the present invention;  
     [0015]FIG. 6 is a perspective view illustrating an alternative battery system, in accordance with one embodiment of the present invention;  
     [0016]FIG. 7 is a sectional view illustrating the battery system under normal temperature and pressure conditions, in accordance with one embodiment of the present invention;  
     [0017]FIG. 8 is a sectional view illustrating the battery system under elevated temperature and pressure conditions, in accordance with one embodiment of the present invention;  
     [0018]FIG. 9 is a sectional view illustrating the battery system under elevated temperature and pressure conditions, in accordance with one embodiment of the present invention;  
     [0019]FIG. 10 is an exploded view of an alternative battery system, in accordance with an alternative embodiment of the present invention;  
     [0020]FIG. 11 is a perspective view of the assembled battery system depicted in FIG. 10, in accordance with an alternative embodiment of the present invention;  
     [0021]FIG. 12 is a sectional view taken along line  12 - 12  of FIG. 11, in accordance with an alternative embodiment of the present invention;  
     [0022]FIG. 13 is a sectional view illustrating the battery system under normal temperature and pressure conditions, in accordance with an alternative embodiment of the present invention;  
     [0023]FIG. 14 is a sectional view illustrating the battery system under elevated temperature and pressure conditions, in accordance with an alternative embodiment of the present invention; and  
     [0024]FIG. 15 is a sectional view illustrating the battery system under elevated temperature and pressure conditions, in accordance with an alternative embodiment of the present invention.  
     [0025] The same reference numerals refer to the same parts throughout the various Figures.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0026] The following description of the preferred embodiments concerning the present invention is merely exemplary in nature, and is not intended to limit the invention or its application or uses. Furthermore, the present invention is not limited to any particular temperature or pressure range, and is intended to be practiced under any number of different temperature and pressure ranges. Moreover, while the present invention is described in detail below with reference to lithium-ion polymer batteries, it will be appreciated by those skilled in the art that the present invention is clearly not limited to only those specific types of batteries, but may be used with other batteries where internal gas pressures build to elevated levels.  
     [0027] Referring to FIGS. 1 through 2, a conventional lithium-ion polymer battery  10  is shown having a rectangular prismatic shape (e.g., substantially box-shaped). The battery  10  includes an electrochemical cell  12  which is also of a rectangular prismatic shape. The electrochemical cell  12  is encapsulated in a flexible packaging material  14  (e.g., an aluminum foil pouch) having essentially the same shape as the electrochemical cell  12 . Pouch  16  may be formed from two separate and substantially complementary sections  18  and  20  that are intended to be brought together to encapsulate electrochemical cell  12 . Alternatively, pouch  16  may be formed from a unitary blank member (not shown) that is folded along one edge, with the two portions then being brought together to encapsulate electrochemical cell  12 . As previously noted, one or more edges of pouch  16  are then brought together and generally heat sealed to form a flange-like structure  22  extending along the periphery of the battery structure. Pouch  16  is sealed around electrode tabs  24  and  26  while permitting tabs  24  and  26  to extend from pouch  16 , thereby permitting the battery  10  to be in electrical communication with an external load (not shown).  
     [0028] Referring to FIGS. 3 through 6, a battery system  100  in accordance with one embodiment of the present invention is shown. System  100  includes an electrochemical cell  102  and a container system  104  which encapsulates or envelopes electrochemical cell  102 . A pair of electrode tabs  102   a  and  102   b , respectively, corresponding to the anode and cathode (not shown), are in electrical communication with electrochemical cell  102 , and extend outwardly away from electrochemical cell  102 . Container system  104  generally at least encapsulates a portion of each electrode tab  102   a ,  102   b , while permitting each electrode tab  102   a ,  102   b  to be in electrical communication with an external load (not shown).  
     [0029] As will be further discussed herein below, the container system  104  includes a vent for expelling gas generated or evolved within the container system  104 . The vent can be in the form of an aperture through a wall of the container system  104  and/or could be formed at the intersection of two or more walls or surfaces of the container system  104 . A temperature sensitive sealant in communication with the vent substantially seals the vent, and unseals the vent to allow for venting of gas evolved within the container system  104  when the operational temperature of the battery system  100  reaches a temperature that corresponds to a minimum temperature at which gas is generated or evolved within the container system  104 . Preferably, the minimum temperature corresponds to a temperature at which “excessive” volumes of gas evolve within the container system  104 . As used herein, the phrase “excessive volume of gas” means any volume of gas which, if left unvented, would affect the performance of the battery and/or would affect the structural integrity or result in a failure of the container system  104 . Furthermore, the term “operational temperature” means the temperature of the battery system  104 , which may result from either thermal energy emitted by the battery system  104  and/or thermal energy absorbed by the battery system  104  from its surroundings.  
     [0030] In one preferred embodiment, container system  104  includes two complementary container members  106   a  and  106   b . The container members  106   a ,  106   b  are complementary and are substantially identically configured. Each container member  106   a , 106   b  includes a body  108   a  and  108   b , respectfully, having a substantially rectangular-shaped recess  110   a , 110   b  for receiving electrochemical cell  102  (see FIGS. 3 and 6). Flange-like members  112   a , 112   b  extend along the periphery of each container member  106   a  and  106   b , respectively.  
     [0031] Alternatively, container system  104  can be constructed of a single blank, as shown in FIG. 6, which is then folded along one or more edges to form a suitable container system. In either case, the container system  104  may be formed from a number of different materials suitable for containing the electrochemical cell  102  including, by way of a non-limiting example, metallic materials such as aluminum foil or the like.  
     [0032] Using conventional methods, system  100  can be assembled by bringing the container members  106   a , 106   b  together to encapsulate the electrochemical cell  102  such that the respective surfaces  114   a  and  114   b  of flange-like members  112   a  and  112   b , respectively, are in adjacent and abutting co-planar communication. Heat is then applied to members  106   a , 106   b , thus creating a heat seal at the interface of surfaces  114   a  and  114   b . This creates an essentially air-tight and water-tight seal along the entire periphery of surfaces  114   a , 114   b.    
     [0033] The sealant is preferably a sealant material  118 , which is preferably disposed on one or both of flange-like surfaces  114  and  114   a  such that when system  100  is fully assembled, the flange-like member surfaces  114   a , 114   b  form a vent therebetween, and the sealant material  118  substantially and selectively seals the vent. In this embodiment, the sealant material  118  is preferably disposed between the flange-like surfaces  114   a , 114   b . Although the sealant material  118  is shown as being applied only in discrete areas on the flange-like surfaces  114   a , 114   b , it should be noted that sealant material  118  can also be applied as a continuous layer along one or both flange-like surfaces  114   a , 114   b . Sealant material  118  can be applied in any number of ways, such as rolling, spraying, dipping, painting, inking, and the like. Once sealant material  118  is applied, the flange-like surfaces  114   a , 114   b  are brought together such that the container members  106   a , 106   b  encapsulate the electrochemical cell  102 . Sealant material  118  adheres to surfaces  114  and  114   a , thus creating an essentially air-tight and moisture-tight seal, and fixedly attaches the container members  106   a ,  106   b  to one another.  
     [0034] The sealant material  118  also provides a means for permitting gas pressure to escape from the interior of the packaging system  104 . As previously noted, excessive levels of heat generated inside the packaging system  104  typically coincides with the production of gas, which builds up inside the packaging system  104  because there is no means of venting or expelling the gas. Once the pressure builds up to a certain point, there is generally a decrease in the structural integrity and/or a failure of the packaging system  104 , which may damage or destroy the battery  100 , and typically damages the electronic device housing the battery  100 . The present invention overcomes this problem by employing a sealant material  118  that is a substantially “heat-sensitive” material. By “heat-sensitive” as that term is used herein, it is meant that the sealant material  118  is a substantially hard solid material below a preferably pre-determined temperature or temperature range, and is a substantially soft semi-solid or liquid material above a preferably pre-determined temperature or temperature range.  
     [0035]FIGS. 7 through 9 illustrate the operation of the present invention. FIG. 7 illustrates the present invention wherein the sealant material  118  is subjected to a temperature at which the sealant material  118  is a substantially hard, solid material. FIG. 8 illustrates the present invention as the operating temperature of the battery system  100  begins to elevate above the softening or melting point of the sealant material  118 , and the sealant material  118  starts to become semi-solid or possibly liquid. The gas pressure begins to push or displace the semi-solid or possibly liquid sealant material  118  outwardly towards the ambient atmosphere, and/or may push or displace the semi-solid or possibly liquid sealant material  118  upwardly and/or downwardly against one or both of the flange-like surfaces  114   a , 114   b . Once the semi-solid or possibly liquid sealant material  118 , or a portion thereof, is displaced, the gas pressure G inside packaging system  104  passes to the ambient atmosphere (i.e., between surfaces  114   a  and  114   b ), thus permitting the venting of the gas pressure. It should be noted that the gas pressure can be vented simultaneously at other locations along the peripheral seal as well, and not just at the particular location depicted in FIG. 9.  
     [0036] The material that comprises sealant material  118  is preferably substantially non-reactive with respect to the materials employed to manufacture the packaging system  104  and the electrochemical cell  102 , or any other structures of the battery system  100  not specifically discussed. The reason for this is that if sealant material  118  reacts with any of the afore-mentioned objects or materials, it could adversely affect the performance of battery system  100  or the performance of sealant material  118 , as described above.  
     [0037] As noted above, the sealant material  118  employed is preferably substantially heat-sensitive, capable of fixedly attaching the container members  106   a , 106   b  together to encapsulate the electrochemical cell  102 , and is non-reactive. As discussed above, another consideration in choosing a suitable sealant material  118  that is solid at or below a certain temperature or temperature range, and is semi-solid or liquid at or above another temperature or temperature range. Choosing an adhesive material that softens at too low a temperature range can potentially result in the premature softening or melting of the seal, i.e., when the internal gas pressure is relatively low and non-threatening. Conversely, choosing an adhesive material that softens at too high of an elevated temperature could potentially cause the seal around the packaging system (or any other portion of the battery system  100 ) to catastrophically fail, i.e., when the internal gas pressure is elevated above a desired pressure.  
     [0038] It should also be recognized that the battery operating temperature and gas pressure do not necessarily have a linear relationship with respect to the operating parameters of an electrochemical cell. For example, some electrochemical cells normally operate at relatively elevated internal temperatures, without any significant concurrent production of excessive amounts of gas. Conversely, some electrochemical cells normally operate at relatively low internal temperatures, which and produce excessive amounts of gas at a relatively lower temperature. Therefore, the sealant material  118  is preferably chosen to be a material that has a softening point temperature corresponding to the minimum temperature at which an “excessive” volume of gas is produced or generated within the particular container system  104 . By way of a non-limiting example, with respect to lithium-ion polymer batteries, internal gas pressures in the range of about 15 to about 28 or more pounds per square inch (psi) are preferably avoided, and thus, choosing sealant material  118  having a softening point temperature corresponding to the temperature(s) at which gas is formed in amounts corresponding to this particular pressure range would be preferred in order to relieve the gas pressure. In accordance with a preferred embodiment, it is preferred to employ a sealant material  118  that has a softening point temperature corresponding to the temperature(s) at which an a sufficient amount of gas is formed to produce an internal gas pressure in the range of about 22 to about 28 or more psi. Although the temperature at which the generation of sufficiently elevated enough gas pressures will occur will vary, as previously noted, it is preferred to employ a sealant material  118  that has a softening point in the range of about 100 to about 200 degrees Celsius.  
     [0039] Preferably, the sealant material  118  is one or more thermoplastic or thermoset polymeric materials, preferably one or more thermoplastic adhesive materials. Preferred thermoplastic materials include, but are not limited to, polyolefins. Preferred polyolefins include, but are not limited to polyethylene. A preferred polyethylene is low molecular weight polyethylene, also commonly referred to as polyethylene wax.  
     [0040] Another preferred adhesive material is selected from the group of adhesives commonly referred to as “hot-melt” adhesives. Hot-melt adhesives are 100% solids that, in the broadest sense, include all thermoplastics. Materials that are primarily used as hot-melt adhesives include ethylene/vinyl acetate copolymers (EVA), polyvinyl acetates (PVA), as previously mentioned, polyethylene (PE), amorphous polypropylene, block copolymers such as those based on styrene and elastomeric segments or ether and amide segments (i.e., thermoplastic elastomers), polyamides, and polyesters. Other adhesive materials having the same or similar chemical and/or physical properties and characteristics of hot-melt adhesives are also envisioned to be suitable to practice the present invention.  
     [0041] In general, hot-melt adhesives are solid at temperatures below 79° C. (175° F.). Ideally, as the temperature is increased beyond this point, the material rapidly softens or melts to a low-viscosity fluid that is flowable and mobile. Upon cooling, the adhesive sets rapidly (i.e., hardens). Because these materials are thermoplastic, the melting/softening-resolidification process is repeatable with the addition and removal of the required amount of heat.  
     [0042] To select a suitable adhesive material, the minimum temperature or temperature range within which unacceptable or undesirable gas pressure levels form inside the packaging system is first determined. Next, a sealant material  118  is chosen which softens or melts at a temperature or within a temperature range substantially corresponding to (or even below) the temperature range during which “excessive” volumes of gas evolve inside the packaging system  104 . In this manner, each time gas pressure builds to sufficiently elevated enough levels, the sealant material  118  will soften, thus allowing the gas pressure to safely and automatically vent to the ambient atmosphere. Thereafter, when the temperature of the battery system  100  drops to below the softening point temperature of the sealant material  118 , the sealant material  118  hardens, thus substantially reforming the original seal.  
     [0043] Referring to FIGS. 10 through 12, an alternative battery system  200  is shown, in accordance with an alternative embodiment of the present invention. As with the previously described embodiment, system  200  primarily includes an electrochemical cell  202  and a container system  204  which is intended to encapsulate or envelope electrochemical cell  202 . A pair of electrode tabs  202   a  and  202   b , respectively, corresponding to the anode and cathode, are in electrical communication with electrochemical cell  202 , and extend outwardly away from electrochemical cell  202 . Container system  204  generally at least encapsulates a portion of the electrode tabs  202   a , 202   b , respectively, while permitting the electrode tabs  202   a , 202   b  to be in electrical communication with an external load (not shown).  
     [0044] Again, container system  204  includes two complementary container members  206   a  and  206   b . Each container member  206   a , 206   b  includes a body  208  having a substantially rectangular-shaped recess defined by surface  210   a , 210   b  for receiving electrochemical cell  202  (see FIGS. 10 and 12). Each container member  206   a , 206   b  includes a flange-like member  212   a  and  212   b , respectively, extending along the periphery thereof. Alternatively, container system  204  can be constructed of a single blank which is then folded along one or more edges to form a suitable container system, as previously shown in FIG. 6.  
     [0045] As shown in FIG. 10, apertures or vents  216  and  218 , respectively, extend through substantially planar surface  220  and  222 , respectively, of container bodies  208   a  and  208   b , respectfully. Preferably, vents  216  and  218  are substantially aligned with one another. While the vents  216 , 218  are shown in the Figures as being circular in shape, other geometrical shapes may be employed, and shape of the vents  216 , 218  is not thought to be critical to the invention.  
     [0046] Each vent  216 , 218  is covered by a non-reactive vent cover  224   a  and  224   b , respectively. The vent covers  224   a , 224   b  can be manufactured from any number of suitable materials, such as metals (e.g., metallized MYLAR™, aluminum foil, and so forth), thermosets, thermoplastics, and the like. It is most preferred that the vent covers  224   a , 224   b  employed be inert with respect to the components of electrochemical cell  202  or packaging system  204 .  
     [0047] In order to secure vent covers  224   a , 224   b  over vents  216  and  218 , respectively, it is necessary to employ a sealant material  226 . Again, it is preferred to employ the substantially non-reactive and heat-sensitive adhesive materials previously discussed in relation to the first embodiment, i.e., polymeric materials such as thermoplastic and thermoset materials, and especially those materials characterized as hot-melt adhesives.  
     [0048] By way of a non-limiting example, sealant material  226  is applied around the periphery of each vent  216 , 218  in accordance with any number of suitable methods, such as, but not limited to, rolling, spraying, dipping, painting, inking, and the like. Once sealant material  226  has been applied, each vent cover  224   a , 224   b  is then placed on top of sealant material  226 , thus covering their respective vents  216  and  218 , respectively, and establishing a substantially air-tight and moisture-tight seal about each vent  216  and  218 , respectively. Alternatively, cover material  224  can be pre-fabricated to include a sealant material thereon or embedded therein, if so desired. The purpose of vents  216 , 218  is to allow the venting of elevated pressure gas there through, instead of or in addition to allowing the elevated pressure gas to vent through the peripheral seal, as previously described in the first embodiment.  
     [0049]FIGS. 13 through 15 illustrate the operation of the alternate embodiment of the present invention. FIG. 13 illustrates the present invention wherein the sealant material  216  is subjected to a temperature at which the sealant material  216  is a substantially hard, solid material. FIG. 14 illustrates the present invention as the temperature begins to elevate above the softening or melting point of the sealant material  216 , and the sealant material  216  starts to become semi-solid or possibly liquid. The gas pressure begins to push or displace the semi-solid or possibly liquid sealant material  216  outwardly towards the ambient atmosphere, and/or may push or displace the semi-solid or possibly liquid sealant material  216  upwardly against the respective vent cover  224   a , 224   b , and/or downwardly against the respective container body planar surface  220 , 222 . Once the semi-solid or possibly liquid sealant material  216 , or a portion thereof, is displaced, the gas pressure G inside packaging system  204  passes to the ambient atmosphere, thus permitting the venting of the gas pressure. It should be noted that the gas pressure can be vented simultaneously at vent  218  as well, and not just at vent  216  as depicted in FIG. 15.  
     [0050] It is further envisioned that the present invention can be practiced by simultaneously employing both a peripheral seal, as described in relation to the first embodiment, as well one or more vents or apertures, as described in relation to the alternative embodiment. In this regard, a battery system could have a sealant material disposed in between the peripheral seal, as well as a pair of vent holes which are covered with cover members releasably held in place with a sealant material.  
     [0051] The foregoing description is considered illustrative only of the principles of the invention. Furthermore, because numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and process shown as described above. Accordingly, all suitable modifications and equivalents that may be resorted to that falls within the scope of the invention as defined by the claims that follow.