ENCLOSED CELL HAVING SAFETY VALVE MECHANISM AND FABRICATING METHOD OF THE SAME

Abstract: The present invention relates an enclosed cell and fabri­ cating methods of the cell which has a safety valve mechan­ ism for releasing gas from inside the cell when an internal pressure of the cell rises to an excessive degree. The charac­ terizing feature of the invention resides in the use of a resin packing as sealing element, junctions between the resin packing and a metal lid and between the resin packing and a polar terminal being closed tight by thermal fusion. This construction assures reliable sealing in normal state, and permits the packing to break with an excessive rise in the internal pressure thereby to release gas from inside the cell. The cell of this invention is readily fabricated by injection molding or by a hot press process. Therefore, the enclosed cell according to this invention is fabricated very rationally and inexpensively, assures excellent sealing and safety, and has wide application.

Description: Background of the Invention (1) Field of the Invention The present invention relates to an enclosed cell such as an alkaline storage battery or a lithium battery having a safety valve mechanism and methods of fabricating same. The safety valve mechanism is operable to release gas from inside the cell when an internal pressure of the cell rises to an excessive degree. (2) Description of the Prior Art With an enclosed cell such as an alkaline battery or a lithium battery, it is generally an important matter from the safety point of view that its opening is sealed tight. An insufficient sealing results in liquid leakage. In the case of lithium cell in particular, the insufficient sealing leads to a further problem of deterioration in cell performance. This arises from the reaction of the lithium anode with the moisture in ambient air entering the cell. The reaction produces a passivation film on the lithium anode surface which increases the internal resistance of the cell. Various cells have heretofore been proposed with a view to improvement in sealing reliability. Fig. 1 of the accompanying drawings illustrates one such example of enclosed cell, wherein the improvement resides in a hermetic seal structure comprising a glass or ceramic insulator a. More particularly, this enclosed cell comprises an outer canister b acting as one of the polar terminals, a metal lid c fused to the outer canister b such as by laser welding, and a cylindrical pin d inserted into and fixed to the insulator a to act as the other polar terminal. The outer canister b includes a thin wall portion e which breaks when the internal resistance of the cell rises as a result of misuse or in an abnormal environment. The internal pressure is released to the ambient through the breakage, thereby to prevent the cell from bursting. However, this hermetic seal structure generally is costly because the insulator a is formed of glass or ceramics. It is also difficult to control the thickness t of the thin wall portion e. This impedes valve operating pressure setting, which is contrary to full assurance of safety. Fig. 2 illustrates a second example of enclosed cell which is known from GB-A-2 138 200 and which is an enclosed cell according to the preamble part of claim 1. To improve the sealing this cell discards the hermetic seal structure which is costly and involves difficulties from the point of view of manufacturing techique, in favor of using an insulating packing f formed of a resin (see the Japanese utility model application laid open under No. 60-22753). In this enclosed cell, a second terminal d having a T-shape section is inserted into the packing f and calked by means of a metal washer g fitted on a lower position of the termiral d. This example includes a metal lid c secured to an outer canister b by laser welding or the like as in the first known example. The resin packing f melts and forms an opening (not shown) when the internal pressure of the cell increases or the cell temperature rises to an excessive degree. The gas in the cell is released to the ambient through this opening, thereby to check the internal pressure increasing and prevent the cell from bursting. In this enclosed cell, however, the washer g has an outside diameter g greater than the diameter i of a bore defined in the metal lid c, and the bore may be closed by the washer g when the terminal d moves in a direction of arrow j with a rise in the internal pressure of the cell. Thus, the washer g may obstruct the release of the gas from inside the cell. This cell fails to assure safety when used in a severe environment, for example at high temperature. Summary of the Invention An object of the present invention is to provide a enclosed cell assuring a high degree of safety, with a reliable sealing, which is simple in construction, and includes a safety valve mechanism having excellent operability. This object is achieved by an enclosed cell having the features given in claim 1. According to the construction, according to the invention, the resin packing adheres to the metal elements with a great adhesive strength after the packing is thermally fused thereto in proper conditions, which maintains a reliable sealing at normal times. This resin packing deforms or breaks to release gras from inside the cell on a short-circuit or other abnormal occasion when the cell becomes heated or its internal pressure rises to an excess. In other words, the resin packing acts as the safety valve mechanism and, as noted above, deforms or breaks to outwardly release gas from inside the cell when the internal pressure of the cell exceeds a predetermined value. This effect is promoted where the through bore is formed in the metal lid to be continuous with at least one cutout. The cutout renders the metal lid readily deformable in regions thereof adjacent the through bore as a result of an excessive pressure rise inside the cell. This permits the safety valve operating pressure to be controlled with ease, thereby improving the safety aspect more. In view of the sealing performance and other requirements, the packing material is selected from resins having polar groups, such as polyamide 11, polyamide 12, modified polypropylene and modified polyethylene. Another object of the invention is to provide a method of rationally fabricating an enclosed cell having a safety valve mechanism having an excellent operability as noted above. Accordingly, a method of fabricating an enclosed cell having a safety valve mechanism according to the invention comprises the steps given in claim 2. In the above fabricating method, the packing is formed by injection molding with the metal lid and polar terminal placed in a space surrounded by a first and a second dies. This permits the packing formation and thermal fusion to be carried out simultaneously, which leads to a simplified fabricating process and to low manufacturing cost. The safety valve operating pressure may be set to various values by changing the resin packing thickness, its area subjected to the pressure, and the kind, grade and the like of resin material for forming the packing. The safety valve operating pressure is also ajustable by increasing or decreasing the width, lenght and number of cutouts formed continuous with the through bore of the metal lid. Particularly the dimensions of the cutouts may readily be adjusted in the manufacturing process, which has the advantage of varying the valve operating pressure with ease. The enclosed cell constructed as above assures a high degree of safety and has excellent operational response since the cell is sealed reliably and its safety valve mechanism is properly operable under severe conditions. Moreover, this cell has a simple and inexpensive construction and hence an immense practical utility. The cell fabricating method is highly rationalized for low cost production. Brief Description of the Drawings These and other objects or features of the present invention will become apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings, in which:- Fig. 1 is a sectional view of a sealing portion of a first example of known enclosed cell, Fig. 2 is a sectional view of a sealing portion of a second example of known enclosed cell, Fig. 3 is a sectional view of an enclosed cell having a safety valve mechanism according to a first embodiment of the present invention, Fig. 4 is an explanatory view illustrating a fabricating process of a safety valve system for the cell shown in Fig. 3, Fig. 5 is a plan view of a metal lid according to a second embodiment of the invention, Fig. 6 is a section taken on line A-A of Fig. 5, Fig. 7 is a view showing storage characteristics of the enclosed cell according to the present invention, and Fig. 8 is a sectional view of a metal lid according to a further embodiment of the invention. Detailed Description of the Preferred Embodiments In the following description, like parts are designated by like reference numbers throughout the several drawings. Fig. 3 is a sectional view of a manganese dioxide-lithium battery which is one example of enclosed cell according to the present invention. This battery comprises an outer canister 1 acting as a positive terminal, a metal lid 2 approximately of a dish shape fused over an entire circumference to the outer canister 1 by laser welding or the like and defining a through bore 14 centrally thereof, an insulating packing 3 formed of a resin having polar groups such as modified polypropylene or polyamide 11, polyamide 12 or the like for adhering the metal lid 2, a negative terminal 5 having a substantially T-shaped section and extending through a center bore 4 of the packing 3, and an electrode assembly 6. The electrode assembly 6 includes a cathode having manganese dioxide as its active material, an anode having lithium as its active material, and a bag-like separator 9 interposed between the anode and cathode 7, 8. The electrode assembly, together with an unillustrated electrolyte, constitutes generating elements. The anode 7 is electrically connected to the outer canister 1 under a certain contact pressure, while the cathode 8 is electrically connected to the negative terminal 5 through a negative polar tab 10. Number 13 indicates an insulating sleeve for preventing short-circuit of the electrodes in the battery. In order to provide excellent sealing, this enclosed cell has rigid junctions achieved by thermal fusion between the metal lid 2 and insulating packing 3 and between the insulating packing 3 and negative terminal 5, respectively. Fig. 4 illustrate a process of injection molding which is one example of means for thermally fusing these junctions. Number 15 indicates an upper die and number 16 indicates a lower die. Polyamide 12 which has a particularly good adhesive property with respect to metals is employed for forming the insulating packing 3. In this drawing, the metal lid 2 and negative terminal 5 are first placed in a space 17 between the upper and lower dies 15, 16, and then polyamide 12 melted at 230°C is injected under a pressure of about 300kg/cm² (2.94 x 10⁴ KPa) into an injecting bore 18 defined in the upper die 15 as shown by arrows B. The injected polyamide 12 fills the space 17 and forms a resin packing. In the course of hardening in this process, the molten polyamide 12 adheres tight to the metal elements, namely the metal lid 2 and negative terminal 5, and becomes fused thereto. This process of fusing the packing to the metal lid 2 and negative terminal 5 simultaneously with the packing formation, improves productivity and lowers manufacturing cost. Number 19 indicates heaters embedded in the dies 15, 16. These heaters 19 heat the dies 15, 16 which in turn heat the space 17, negative terminal 5 and metal lid 2 to a predetermined temperature. Table 1 shows the results of a safety valve operating pressure test conducted on the first and second known enclosed cells noted hereinbefore and the cell according to the present invention (first embodiment) fabricated by the above thermal fusion process. Each cell had the outer canister and metal lid formed of a stainless steel sheet having a 0.3mm thickness. The thin wall portion e of the first known cell (shown in Fig. 1 ) was formed into a 0.1mm thickness t. The valve operating pressures were measured by sealing the cells with the generating elements excluded from the cells. The cells were internally pressurized from atmospheric pressure up to 100kg/cm² (9.8 x 10³ KPa) at the rate of 2kg/cm² (1.96 x 10² KPa) per second. The test was conducted at 110°C atmospheric temperature and by using 10 samples for each type of cell. The two remaining samples of the first known cell broke at the laser welded position under the pressure of about 70kg/cm². Table 2 shows the result of a similar test in which the cells were internally pressurized at the rate of 20kg/cm² (1.96 x 10³ KPa) per second. This test was conducted at the same atmospheric temperature and in respect of the same number of samples as in the foregoing test. The two remaining samples of the first known cell burst because of inoperative valves. The three remaining samples of the second known cell broke out at the weld junction between the outer canister and metal lid under the pressure of 70-80kg/cm² (6.86 x 10³ -7.84 x 10³ KPa). In the latter case, the valves operate when the pressure rises to 50kg/cm² (4.9 x 10³ KPa) but fail to cope with a further release of the internal gas at the higher pressures. As seen from Tables 1 and 2, it has been confirmed through the tests that the enclosed cell according to the present invention has its safety valve mechanism acting to prevent bursting of the cell even under severe conditions as above. Then the cells were tested by mounting therein the generating elements including the anode 7 and cathode 8. The cell samples were charged with 6V first, and the safety valve of every sample operated to prevent bursting and other trouble. When the samples were charged with 12V, however, two samples of the first known cell and three of the second known cell burst though no sample of the cell according to this invention (first embodiment) burst. A further, heating test was conducted by placing each cell sample 5 cm from an acetylene burner. None of the cell samples according to this invention burst thanks to the safety valve mechanism coming into operation, but two samples of the first known cell and one sample of the second known cell burst. Table 3 shows the results of a drop test carried out to compare the strengths of the enclosed cell according to this invention (first embodiment) and the first known cell having the hermetic seal which is considered to provide an excellent sealing. Thirty samples were used for each cell, and the number of leaking samples were counted after dropping them. The test was conducted by throwing each sample ten times in a selected direction from a height of 1.5m to a concrete surface. The hermetic seal had an insulator a (Fig. 1) formed of glass. As seen from Table 3, the first known cell must be handled with care since the insulator such as of glass or ceramics used in hermetic sealing is hard and brittle and therefore vulnerable to impact, whereas the first embodiment of the invention is easy to handle since it is sealed with the resin packing which is strong against impact. On the other hand, a helium leak test showed substantially the same leak value for the two types of cells. Table 4 shows its measurement results. Thus, the cell according to the first embodiment of the invention and the first known cell are equal with respect to the sealing performance under normal circumstances. Table 5 shows storage characteristics of the cells. The number of samples used was 100. In the case of enclosed cell, cell performance deteriorates after a long storage time due to the moisture of ambient air entering the cell. As seen from table 5, the enclosed cell according to this invention retains approximately the same internal resistance after a storage period as the first and second known cells, and remains just as well sealed as the prior art cells. Where lithium is used for the anode as in this manganese dioxide-lithium battery lithium ions in the cell react with silicone dioxide constituting the principal component of glass, thereby to promote disintegration of the glass. This is responsible for the four leaking samples of the first known samples. In contrast, the packing material used in the first embodiment of the invention does not react with lithium ions, and therefore no leakage takes place. The cell according to the first embodiment is well sealed as described above and can dispense with the washer mounted in the second known cell (Fig. 2), which means a reduction in the number of components. With the removal of the washer, a special safety valve structure is no longer required since the negative terminal will disengage from the cell by the gas pressure when, for example, the internal resistance of the cell rises under abnormal circumstances. Consequently, the invention has realized a cell having a simple construction and a high degree of safety. Figs. 5 and 6 illustrate a second embodiment of the invention which includes an improved metal lid 2. This lid 2 has a dish-like shape and perforated with a center through bore 14 which is continuous with four cutouts 20 to define a cruciform bore 21 (opening) in plan view. A safety valve mechanism operating test was carried out on a cell having this improved metal lid 2. In this test the cell having the metal lid 2 in the first embodiment was used for comparison purposes. Both of these cells had a 17mm outside diameter D and a 33.5mm height H, and their outer canisters 1 and metal lids 2 were formed of a 0.3mm stainless steel sheet (see Fig. 3). The through borer 14 in each lid had a 2.3mm diameter, and each of the four cutouts 20 in the second embodiment had a 1.8mm lenght L and a 0.5mm width M (see Fig. 5). Table 6 shows the results of this test, i.e. valve operating pressure measurements. In the test, the valve operating pressure was measured at room temperature and at 100°C, using cells of 1800mAh nominal capacity charged in a constant temperature oven with 6V constant voltage. The cells used were those having the same storage characteristics. It will be understood from these results that, although the second embodiment has the same sealability as the first embodiment, the second embodiment is responsive to a very low valve operating pressure and the valve operating pressure therefor has a small range of variation with relation to temperature variations. Table 7 shows response time of the cells with the valve operating pressure raised to 30-40kg/cm², that is the time taken from the point of time at which the pressure reaches the set value till the point of time at which the resin breaks and a valve operation takes place. The through bore 14 in the lid of the first embodiment had a 3.5mm diameter and that in the lid of the second embodiment had a 2.3mm diameter. The numbers of samples are 78 for the second embodiment and 284 for the first embodiment. These results prove that the second embodiment has high valve operating precision with a short response time, i.e. excellent response, and a small response time distribution. Next, the internal resistance of these cells was measured by 1KHz alternating current process and storage characteristics were compared. Fig. 7 is a graph showing the results in comparison with the characteristics of the second known cell (Fig. 2). In this drawing, the solid line respresents the cell according to the second embodiment having a 2.3mm through bore diameter, a 1.8mm cutout length L and a 0.5mm cutout width M (Fig. 5), the two-dot and dash line represents a cell according to the first embodiment -1- having a 3.5mm through bore diameter, the dot and dash line represents a cell according to the first embodiment -2- having a 2.3mm through bore diameter, and the broken line represents the second known cell shown in Table 5. The test was conducted at an atmospheric temperature of 60°C and a humidity of 90%. The test results prove that the second embodiment, while having the same valve operating pressure, 30-40kg/cm² (2.94 x 10³ - 3.92 x 10³ KPa), as the first embodiment -1-, is effective with respect to the cell sealing in that it restrains the internal resistance rise by means of the opening of the metal lid which has a substantially diminished size for defining the cutouts. On the other hand, the first embodiment -2- having the same through bore in diameter size as the second embodiment is comparable with the latter with respect to the cell sealing. However, the second embodiment which includes the cutouts may be set to a lower valve operating pressure, and therefore may readily be provided with a desired safety valve mechanism. Thus, it has been confirmed that in any case the enclosed cells according to the present invention have a safety valve mechanism of better characteristics than that of the known enclosed cells. Furthermore, it has been confirmed through a test conducted by Applicant that the operating pressure for the safety valve mechanism of this invention is, as distinct from the prior art, variable by changing the adhesion thickness of the resin packing 3 with respect to the metal elements, i.e. metal lid 2 and negative terminal 5. Tables 8 and 9 show operating pressure characteristics obtained by changing the adhesion thickness T in Fig. 3 to 0.1mm, 0.2mm, 0.3mm and 0.4mm. In Fig. 3, dimension D1 is 4mm, D2 is 7mm, and D3 is 9mm. The test of Table 8 used polyamide 12 as the packing material and the test of Table 9 used modified polyprolylene. As seen from Tables 8 and 9, the packing 3 having the adhesive thickness T of about 0.4mm results in a low operating pressure, enabling an appropriate pressure setting. In the case of modified polypropylene, which has a low material strength and a low adhesive strength than polyamide 12, further lowers the valve operating pressure by about 20% and hence provides for a safety valve of the cell having satisfactory functions. While the second embodiment is provided with the cruciform bore 21 (opening), it has been confirmed that the bore 21 including additional cutouts 20 stabilizes the operating pressure even further. Table 10 shows a comparison in the valve operating pressure between the cruciform bore 21 (opening) and a bore (opening) having a shape of *. It will be understood from the above data that the bore (opening) having the * shape results in a reduced dispersion of operating pressures and a further improved safety valve of the cell. The valve operating pressure may also be lowered for practical purposes by forming the metal lid 2 to be partially thin as shown in Fig. 8. More particularly, the metal lid 2 of Fig. 8 includes thin wall portions defining a circle of 9mm diameter D4 concentric with the cruciform bore 21 (opening). These thin wall portions have a 0.1mm wall thickness N1 (the remaining portion being 0.3mm thick as in the first and second embodiments). This construction is effective to stabilize the valve operating pressure to 28-35kg/cm² (2.74-3.43 x 10³ KPa), which contributes toward improved quality. The present invention is not limited to the described embodiments. The foregoing embodiments employ the thermal fusion method for forming the packing by injection molding and for fusing the packing to the negative terminal and metal lid at the same time. Instead of this process, the packing may be manufactured beforehand, set in the space between the dies together with the metal lid and negative terminal, and then heated and pressurized by suitable means to effect the thermal fusion. This has the advantage of permitting a general purpose packing to be used as it is for the cell. As another fabricating process, a packing is insert molded in the metal lid, then the negative terminal is inserted into the packing, and finally the metal lid and negative terminal are heated and pressurized by the hot press method or the like thereby to bond with each other. These processes, however, require a vacuum drying step since the resin packing has a water absorption of about 1.5%. Generally, a metal surface has "O" and "OH" bonded thereto, and H₂O is hydrogen-bonded to the"O" and "OH". The resin forming the packing also is hydrogen-bonded to these elements. Since the hydrogen-bonding is weaker than the bonding of "O" and "OH" on the metal surface, the H₂O present between the metal surface and the resin having polar groups would impair good adhesion. It is therefore necessary to dehydrate the packing. Thus, the resin packing is dehydrated by vacuum drying it under a reduced pressure of 4mmHg and at 120°C for about two hours, for example. Then the negative terminal is inserted into the packing in dry atmosphere, which is followed by a hot press heating step for heating the metal lid and polar terminal at 200°C for two seconds, for example. A packing material having no polar group cannot be used for lack of the adhesive property with respect to metals, but any resin may be used only if it has polar groups. However, it is desirable to use modified polypropylene or polyamide 12 or polyamide 11 as described with the foregoing embodiments. As is well known, polyamide 12 and polyamide 11 have the excellent rate proof water penetration, and are well suited where tight contact and high sealing performance are required of the packing. Modified polypropylene is suitable for setting the valve operating pressure low.

Claims: 1. An enclosed cell having a safety valve mechanism, comprising: an outer canister (1) having an opening at one end thereof and acting as a first polar terminal, a generating element disposed in said outer canister (1) and including an electrode assembly (7, 8) and an electrolyte, a metal lid (2) fused to a peripheral edge of the opening of said outer canister (1) containing said generating element, said metal lid (2) defining a through bore (14) centrally thereof and a second polar terminal (5) including a planar portion and a pin component extending from a center of an undersurface of said planar portion, said pin component extending into said through bore (14) without contacting said through bore (14), and a resin packing (3) having polar groups and being placed between said metal lid (2) and said second polar terminal (5) for sealing the cell interior gastight, characterized in that said packing (3) defines seals by thermal fusion thereof to said metal lid (2) and to a peripheral face of said pin component, that with said metal lid (2) is provided at least one cutout (20) and/or with portions having a reduced thickness adjacent to said through bore (14), whereby said packing (3) is deformable and/or breakable to release a gas when the internal pressure of the cell increases to excess. 2. A method for fabricating a cell as claimed in claim 1, characterized by the steps of: placing the metal lid (2) defining the through bore (14), which is to be fused to the outer canister (1), and the polar terminal (5) in a space surrounded by a first die (15) and a second die (16), heating said first and second dies (15,16) to a temperature below the resin melting point, injecting molten resin having polar groups into said space, and hardening the molten resin into said resin packing (3) and thereby fastering it to said metal lid (2) and said polar terminal (5). 3. An enclosed cell as claimed in claim 1, wherein the material of said resin packing (3) is polyamide 11, polyamide 12, modified polypropylene or modified polyethylene.

Inventors: AMAZUTSUMI, TOORU, HARA, MITSUNORI, NAGAO, NOBUHIRO, URUSHIHARA, KANJI
Applicants: SANYO ELECTRIC CO., LTD.
Application year: 1987
Main classification: H01M2
IPC codes: H01M2