Sealed lead-acid battery

Abstract: A sealed lead-acid battery includes a separator made from hydrophilic treated sheet made from synthetic fiber wraps at least either one of positive electrode and negative electrode, and a mat type separator made from fine glass fiber is disposed between the wrapped electrode and its opposite electrode. This structure is adopted in a battery which uses a paste type electrode where an expanded grid having no outer frame on both edges is used, thereby prolonging cycle life even after cycling of charge and deep discharge.

Description: Field of the Invention The present invention relates to a sealed lead-acid battery, and more particularly to a separator thereof which prevents an internal short-circuit and thereby realizing a longer-life battery Background of the Invention In general, a discharge reaction of the lead-acid battery produces lead sulfate (PbSO 4 ) through electrochemical reduction and oxidation of lead dioxide(PbO 2 ) and lead(Pb) in sulfuric acid (H 2 SO 4 ) electrolyte solution; where PbO 2 and Pb are active materials of positive and negative electrodes. The produced PbSO 4 on the positive and negative electrodes is electrochemically reduced as well as oxidated by charging, whereby not only producing PbO 2 and Pb, but also revitalizing H 2 SO 4 . The overall reaction of charge/discharge of the lead-acid battery is described as the following formula (1): The following characteristics are desirable for the separator of the lead-acid battery: 1. Acid-resistant, anti-oxidation and anti-reduction Since strong-acid H 2 SO 4 may be used as electrolyte, and also, electrochemical oxidation as well as reduction is repeated on both the positive and negative electrodes through charging and discharging processes, a chemically stable material from which a harmful substance is not soluble should be selected. 2. High ion-conductivity and no possibility of internal short-circuit due to contact between the positive and negative electrodes Electrolyte is easy to permeate and diffuse through the separator disposed between the positive and negative electrodes, for hydrogen ion H + and sulfate ion SO -2 / 4, both are of electrolytic dissociation in the electrolyte together with active materials PbO 2 and Pb of the positive and negative electrodes, are reaction species, whereby the separator is vulnerable to be permeated as well as diffused with the electrolyte. In order to avoid contact between the positive and negative electrodes, it is desirable for the separator to be long and bent in shape, and to have a micro-porous construction. Particles in the active materials of the positive electrode, by charge/discharge cycling, tend to drop from an electrode grid which is an electric collector and active material holder. Thus caution is desirable to avoid this drop. To be more specific, a micro-porous sheet made of rubber or resin, and pulp or glass-fiber reinforced with resin is used as the separator. A paste type electrode plate process which is in high productivity is widely used, among others, in the lead-acid batteries. When using this type, a synthetic resin sheet having the micro-pore structure, and a glass mat are used for the separator. The glass mat is in contact with both sides of the positive electrode, whereby preventing the active material of the positive electrode from dropping. As a result, the number of internal short-circuits is reduced. This example is substantially effective when the electrode plate is used in a vented type car battery which is exposed to violent external forces such as vibration, shock, and acceleration. Recently, Japanese utility model H07-34555 discloses another idea as follows which further reduces the internal short-circuit; an envelope type separator made from micro-porous synthetic resin sheet comprising: a glass mat of which width is broader than the separator, being laid on the separator, wherein the separator is double backed with thereinside so that the synthetic resin sheet may contact with each other, and both edges where the synthetic resin sheet contacting and the outer both edges where a glass mat contact with each other adhere to each other in order to form an envelope. This separator is also developed to be used in the vented type battery, where the negative electrode is accommodated into this envelope, and the positive electrode is disposed to contact with an outer surface of the glass mat. The electrode group thus formed is incorporated in a cell container, thereby forming the cell. In order to make a maintenance work of the lead-acid battery easy, a sealed lead-acid battery has been widely used, because oxygen (O 2 ) gas generated from the positive electrode by over-charging is eliminated by an oxygen cycle reaction on the negative electrode. The same process is seen also in a sealed nickel-cadmium battery. The separator of the sealed lead-acid battery desirably has the following functions other than the above: (a) O 2 gas generated from the positive electrode during a period between from an end of charging to over-charging of the positive electrode, flows to the negative electrode with ease because of excellent ventilation, and the electrolyte is prevented from being fluid. (b) The electrolyte pertinent to charge/discharge is retained as much as possible around the positive and negative electrodes. In order to turn the electrolyte to non-fluid condition, two methods are available: (1) Retainer method: The electrolyte is absorbed into the positive and negative electrodes as well as the separator so that the electrolyte turns into the pore of solid substance. (2) Gelled method: The electrolyte turns into gell with colloidal silica. The retainer method is now mostly used, and it is sometimes called "Absorbed method" or "Starve method." The retainer method features to adjust a quantity of the electrolyte so that ventilation works in some part when the electrolyte is absorbed into the mat type separator and turned into solid. A mat sheet made from fine glass fiber is widely used as a separator satisfying the above functions. This separator made from the mat type glass fiber is a kind of non-woven cloths by making paper mainly short fibers made of borosilicate glass having a diameter of not more than 1 µ m . Fig. 3 shows a typical structure of an electrode group used in a conventional sealed lead-acid battery. Mat type glass fiber separator 3 is folded to form a U-shape, and negative electrode 1 is inserted therein, then the outer side of the folded separator 3 is nipped by positive electrode 2, and whereby the electrode group is formed. When using the electrodes of the same dimension, a number of plates is increased in step with capacity, and the electrode group is formed with the same manner. In general, a number of negative electrodes 1 is larger than that of positive electrodes 2 by one. The electrode group is accommodated into the cell container, and the most appropriate quantity of electrolyte is poured therein, finally the cell container and its cover are sealed to complete the cell. In this case, the separator 3 is shaped longer than both sides and upper ends of the negative electrode 1 and positive electrode 2 in order to avoid the internal short-circuit. This structure finds no problem when used in a battery of small size, relatively low capacity, and the positive and negative electrodes with a casting grid having an outer frame; however, when used in a car battery or an electric vehicle (EV) battery, they are relatively high capacity, and in a sealed lead-acid battery, the internal short-circuit is sometimes occur, because these batteries are exposed to violent outer forces such as vibration, shock, and acceleration. When an expanded grids which has no outer frame of both edges is used in the electrodes, the internal short-circuit often occurs particularly in cycling of charge and deep discharge, because the active materials expand in step with increasing cycles of charge/discharge, which expands an edge of the positive electrode to overgrow the edge of the separator, and whereby the positive electrode makes contact with the negative electrode. The envelope type separator of which both sides are sealed has been thus proposed in order to solve the above problem; however, it has been a difficult to tie both edges of the separator comprising a mat type sheet made of glass fiber while keeping high productivity. Thus felt type non-woven cloth is adopted, which is made from acid proof and thermoplastic synthetic fiber such as polyester or polypropylene, and fine powder of acid clay as well as fine glass fiber is mixed therein. Namely, the felt-type non-woven cloth of which dimension is large enough to fold the electrodes therein is double-backed to form a U-shape, and both edges thereof is heat-sealed to make the envelope type separator. Then the electrode is inserted into this envelope. An example of this method is disclosed in Japanese Patent Laid-open H07-60676, where the envelope separator is used, which comprises felt-type non-woven cloth made from thermoplastic synthetic fiber such as polypropylene and fine glass fiber evenly dispersed therein. Although this separator has two advantages including (a) retaining electrolyte and (b) good heat-sealing, it has less ability of retaining electrolyte than the mat-type separator purely made from glass fiber, and yet, ventilation is not enough since a diameter of synthetic fiber is in general larger than that of glass fiber. While increasing the glass fiber content in order to improve the retainability of electrolyte, tightness of heat-sealing lowers, i.e. these two factors are in a trade-off relation. Therefore, the separator disclosed in H07-60676 does not work well in the sealed lead-acid battery with the glass fiber content rate of 10-25 wt% defined in H07-60676. SUMMARY OF THE INVENTION A separator made from hydrophilic treated synthetic fiber surrounds at least one of a positive and a negative electrode, and another separator made from fine glass fiber is also used. This structure results in the following advantages: (a) Electrolyte between the positive and negative electrodes does not dry up. (b) The separator made from hydrophilic treated synthetic fiber non woven and thermoplastic cloth can be shaped into an envelope or a tube by a heat-sealing method, therefore, even charge and deep discharge is cycled in a battery using an expanded grid which does not have outer frame, an internal short-circuit is prevented. Thus, the present invention can prolong the life of a sealed lead-acid battery of which capacity is comparably large, such as a car battery and an EV battery. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1(a) - 1(b) are perspective views and Fig. 1(c) is a side view of an electrode group used in the sealed lead-acid battery, respectively, according to Exemplary Embodiments 1 - 3 of the present invention. Fig. 2 compares the cycle lives between a conventional sealed lead-acid battery and a battery according to the present invention. Fig. 3 is a perspective view of an electrode group used in the conventional sealed lead-acid battery. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is now detailed by referring the drawings and tables. Exemplary Embodiment 1 Fig. 1(a) is a perspective view of an electrode group used in the sealed lead-acid battery according to the present invention. In Fig.1(a), negative electrode 1 is located within separator 4 and is shaped like an envelope. The separator 4 may be either hydrophilic treated non-woven cloth is made from the mixture of (a) polyethylene(PE) fiber and (b) polypropylene (PP) fiber. Two pieces of separator 3 (which may be a mat sheet in which the fibers run in random directions) made from fine glass fiber having a diameter of about 0.8 µ m are in contact with both sides of the envelope, and positive electrode 2 is in further contact with the outside thereof, and thereby forming an electrode group. The negative electrode 1 and the positive electrode 2 are electrode plates (for example, paste type) using an expanded grid made from a Pb-Ca group alloy. The non-woven cloth mixed with PE and PP is dipped in fuming sulfuric acid, thereby sulfonating the surface thereof, thus the hydrophilic treatment is provided to the non-woven cloth. After this treatment, the non-woven cloth is washed with water and is dried. Then the non-woven cloth mixed with PE and PP is double backed to form a "U" shape, and facing edges of both faces are heat-sealed to form an envelope. The separator made from hydrophilic treated non-woven cloth has a pore size of 4 - 27 µ m and 0.2 mm in thickness of the cloth, while the mat type separator made from glass fiber is 1.0mm in thickness. These thickness values are measured when a pressure of 20 kg/dm 2 is applied. A plurality of negative electrode 1, positive electrode 2, envelope separator 4 made from hydrophilic treated synthetic fiber non-woven cloth, and mat type separator 3 made from glass fiber are used to produce an exemplary sealed lead-acid battery A in accordance with the present invention, of which the nominal capacity is 60Ah at a 3 hour-rate, and the nominal voltage is 12V having a mono-block with 6 cells. For a comparison purpose, the following two conventional type sealed lead-acid batteries B and C are produced. Battery B comprises a mat-type separator made from glass fiber, of which thickness is 1.2 mm, and Battery C comprises micro porous PE sheet of which the average pore size is 3.0 µ m and the thickness is 0.5 mm. The mat type separator has a thickness of 0.7 mm. Materials other than the separator of batteries B and C are the same as those of battery A. The structure of the three batteries mentioned above are listed in Table 1. Battery type Separator Type Thickness A of exemplary embodiment 1 hydrophilic treated PE, PP non-woven cloth (envelope type), 0.2 mm mat glass fiber 1.0 mm B of conventional type 1 mat glass fiber 1.2 mm C of conventional type 2 micro-porous PE sheet (envelope type) 0.5 mm mat glass fiber 0.7 mm A specified quantity of H 2 SO 4 electrolyte is poured into the above batteries at ordinary temperature, and the battery container is sealed after receiving an initial charge (formation.) Then, each battery is discharged with 1/3 C (20A) constant current at 25°C down to 9.9V. After each battery is charged at 25°C, each battery is discharged again with 2.5C (150A) constant current. The result of this initial capacity test is shown in Table 2. The discharged capacity is measured at a relative value based on Battery B = 100. Battery type Discharge with 1/3C Discharge with 2.5C A 100 100 B 100 100 C Not discharged Not discharged Regarding "charging", a two-step constant current charging method is adopted, i.e. charging each battery with 1/5C (12A) constant current until a terminal voltage reaches 14.4V, then switch the constant current to 1/20C (3A) and charge each battery again for four hours. Table 2 proves that the battery A of the present invention has a performance at a low-rate (1/3C) discharge and a high-rate (2.5C) discharge as much as the battery B having a separator made from mat glass fiber and featuring an excellent discharge performance. On the other hand, the battery C adopting the envelope type separator together with the mat glass fiber separator cannot discharge even at a low-rate (1/3C) after formation in the battery container, and thus data is not measured. The battery C cannot discharge too at the high-rate (2.5C). The reason may be insufficient diffusion of electrolyte between the positive and negative electrodes, for the electrolyte in the separator, which is directly in contact with the positive electrode and is made from micro porous PE sheet shaped in the envelope, dries up. Because the electrolyte becomes starved due to sealing the battery, and the electrolyte is retained by the mat glass fiber separator. The separator used in the battery C is effective for a vented type battery which exists with ample electrolyte; however, it is not recommended for the sealed type battery. The inventors tested the battery A of the present invention and the conventional battery B for a cycle life by cycling the above mentioned charging and the discharging at the low-rate(1/3C), where an end voltage is 9.9V and a test temperature is 25°C. The test result is shown in Fig. 2. The capacity of the battery A reduces a bit until charge/discharge reaches 500 cycles, and then the capacity is reduced slightly. When 80% of the initial capacity (when the cycle hits 700) is reached, the test is ended. In the meantime, battery B which shows approximately the same initial performance as battery A, reduces its capacity remarkably after 200 cycles, and reaches 80% of the initial capacity at 300 cycles, and finally the internal short-circuit occurs at 350 cycles. The battery B thus cannot be recharged. The battery B is disassembled to find that a grid of the positive electrode extends and touches the active material which has overgrown from both edges of positive electrode and negative electrode, and the internal short-circuit occurs. Exemplary Embodiment 2 The same materials as used in Embodiment 1 are used in Embodiment 2, i.e. the envelope type separator comprising the positive and negative electrodes and hydrophilic treated sheet (e.g. non-woven cloth) made from synthetic fiber, and the separator (e.g. mat type) made from fine glass fiber. As shown in Fig. 1(b), the positive electrode 2 which differs from Embodiment 1, is situated in the envelope type separator comprising hydrophilic treated non-woven cloth 4 made from synthetic fiber. The mat type separator 3 made from glass fiber is on both sides of the enevelope type separator, and negative electrode 1 is further in contact with the outside thereof, so that the electrode group can be formed. This electrode group is adopted into an exemplary battery D in accordance with the present invention. The initial capacity and a cycle life of the battery D are approximately the same as those of the battery A. The battery D and the battery B of conventional type 1 are tested for pass-through of foreign matter attached to the electrodes by a vibration test. The foreign matter, a notched piece of the positive electrode 2 having, for example, a size of 0.7 mm, are intentionally left on the electrodes in forming the electrode group when the batteries are produced. The electrolyte is poured, initial charge (formation) is completed, and the battery containers are sealed. After the initial capacity test is completed, the batteries are charged, and the following vibration test is applied to these batteries D and B: Acceleration = 3.5G, Sweep speed = 1 Hz/sec, Frequency range = 10 - 60 Hz (sweep this range repeatedly) In the battery B, an internal short-circuit occurs in approximately 100 hours when the test starts. The battery B is disassembled to find the active material of the positive electrode has passed through the mat type separator made from glass fiber. The active material from the positive electrode permeates into the mat type separator whose thickness is 1.2 mm by its half thickness in depth and in almost the entire area, and the internal short-circuit is found where a notched piece of active material is intentionally left on the positive electrode. On the other hand, the internal short-circuit does not occur in 400 hours after starting the vibration test in the battery D. The battery D is disassembled when 400 hours have passed to find that there is little damage due to the permeation of the active material into the mat type separator. The envelope type separator comprising hydrophilic treated non-woven synthetic fiber may prevent the permeating of active material from the positive electrode. The above embodiment proves that the battery, where the envelope type separator comprising hydrophilic treated non-woven synthetic fiber surrounds the positive or negative electrode therein, and the mat type separator made from glass fiber which is disposed between the opposite electrode and the envelope type separator, can not only maintain the same discharging performance as the conventional battery which uses purely the mat type separator made from glass fiber, but also realizes a reduction of internal short-circuits, and thereby the cycle life of the battery is dramatically prolonged. In Embodiments 1 and 2, either one of the positive or negative electrode is accommodated into the envelope type separator made from hydrophilic treated non-woven cloth made from synthetic fiber; however, if possible, both of the positive and negative electrodes may be accommodated into the above envelope type separator, and the mat type separator made from glass fiber may be disposed between these two electrodes. This structure can further prolong the cycle life. Exemplary Embodiment 3 There is another method to obtain a similar effect as Embodiments 1 and 2: As shown in Fig. 1(c), two positive electrodes 2, three negative electrodes 1, or the same two electrodes having mat type separators 3 made from glass fiber thereon, are inserted into a tubular separator comprising hydrophilic treated non-woven cloth 4 made from synthetic fiber so that each lower end of the two electrodes may be adjacent, then the tubular separator is double-backed to form a "U" shape. Next, the opposite polar electrode to those in the tubular separator is disposed to contact with the inside and outside of the "U" shape. Instead of the opposite polar electrode, the opposite polar electrode accommodated into the envelope type separator comprising hydrophilic treated non-woven cloth made from synthetic fiber may be contacted therewith. An electrode group so structured can produce the same effect as Embodiments 1 or 2. In the above embodiments, a sulfonating method is adopted to the hydrophilic treatment; however, other methods such as graft polymerization, oxidation, and plasma irradiation are available, and as far as those methods can change the surface property of synthetic fiber to a hydrophilic property, those methods also can be adopted. In the above embodiments, the non-woven cloth made from synthetic fiber is used, and the cloth is produced by mixing PE and PP. The material used in this embodiment is not limited to the non-woven cloth, but woven cloth can be also used. Other synthetic fiber materials such as polyvinyl chloride, polyethylene terephthalate, polyethylene, polypropylene, or a mixed product of those materials can be used, as far as the materials satisfy those properties such as acid-proof, withstanding the electrochemical oxidation and reduction, and hopefully thermoplastic. The hydrophilic treated, and thermoplastic cloth made from synthetic fiber can be formed into the envelope type separator with ease by double-backing the cloth to form a "U" shape and then heat-sealing the faced both edges, and also can be formed into the tubular separator with ease by double-backing the cloth to form a "U" shape and heat-sealing the opposite ends to the fold. The thickness of the above cloth is 0.1 - 0.7 mm, and the mat type separator made from glass fiber can be effectively used provided its thickness ranges from 2.0 to 0.5 mm. As detailed in the above embodiments, the positive or negative electrodes wrapped in the envelope type separator comprising the hydrophilic treated cloth made from synthetic fiber is used together with the mat type separator made from glass fiber, thereby reducing the internal short-circuit, and as a result, the cycle life of the sealed lead-acid battery can be prolonged. In this case, paste type electrode using not only cast grid with an outer frame, but also using the expanded grid without an outer frame on both edges, is adopted to the positive and negative electrodes, and the battery is exposed to violent forces such as vibration, shock, and acceleration, and charge and deep discharge. The present invention dramatically improves the performance of the sealed lead-acid battery, and reduces maintenance of car batteries as well as EV batteries.

Claims: A sealed lead-acid battery including a separator which comprises a hydrophilic treated sheet (4) made from synthetic fiber and a further separator (3) comprising glass fiber. A sealed lead-acid battery according to claim 1, including an electrode group (1,2) which comprises: (a) an envelope type separator formed by the hydrophilic treated sheet (4) made from synthetic fiber; and (b) one of a positive electrode (2) and a negative electrode (1) surrounded by said envelope type separator, the further separator (3) being located outside said envelope type separator (4), and another of said positive electrode (2) and said negative electrode (1) being located outside said further separator (3). A sealed lead-acid battery according to claim 1, which comprises: one of a positive electrode (2) and a negative electrode (1) having outer surfaces surrounded by the first hydrophilic treated sheet (4) made from synthetic fiber and second glass fiber sheet material (3), said second glass fiber sheet material (3) being one of (i) double-backed to form a "U" shape, and (ii) two separate glass fiber sheets, with another of said positive electrode and said negative electrode on both sides of said second glass fiber sheet material. A sealed lead-acid battery according to claim 2, wherein the further separator is in the form of a mat made from fine glass fiber double backed to form a "U" shape, or two pieces of said mat being in contact with the outer surfaces of said envelope type separator and further, the other of said positive electrode and said negative electrode being in contact with said mat type separator. A sealed lead-acid battery according to claim 1, wherein (a) the hydrophilic treated sheet separator is formed into a tube, (b) two electrodes of equal polarity inserted into said tubular separator so that the lower ends thereof become adjacent to each other, (c) the further separator (3) having two sections, said two sections being in contact with both outside surfaces and inside surfaces of said tubular separator, and (d) an electrode opposite in polarity to said two electrodes having said two-part further separators on both sides thereof. A sealed lead-acid battery according to claim 5 wherein said further separator (3) is one of (a) a double backed "U" shape member and (b) two mat type separators. A sealed lead-acid battery according to claim 1, wherein the first-mentioned separator (4) comprises a hydrophilic treated sheet made from at least one thermoplastic synthetic fiber selected from the group consisting of polyvinyl chloride, polyethylene terephthalate, polyethylene, polypropylene. A sealed lead-acid battery according to claim 1, wherein the first-mentioned separator (4) comprises a hydrophilic sheet treated by a method selected from the group consisting of oxidation, sulfonation, graft polymerisation and plasma irradiation. A sealed lead-acid battery according to any one of claims 2 to 6 wherein the first-mentioned separator is made from hydrophilic treated and thermoplastic sheet made form synthetic fiber and is shaped in one of an envelope and a tube by heat-sealing. A sealed lead-acid battery according to any one of the preceding claims, wherein the hydrophilic treated separator (4) has a thickness in the range of 0.1-0.7 mm, and said further separator (3) has a thickness in the range of 2.0-2.5mm. A sealed lead-acid battery according to claim 1, 2 or 3, wherein at least one of (a) said first-mentioned separator (4) is non-woven cloth, and (b) said further separator (3) is a mat-type fine glass fiber.

Inventors: ANDO KAZUNARI, HOTTORI TAKASHI, TANIGUCHI AKIHIRO, ANDO, KAZUNARI, HOTTORI, TAKASHI, TANIGUCHI, AKIHIRO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1997
Main classification: H01M2
IPC codes: H01M2, H01M10, H01M4