LAMINATED LEAD ALLOY STRIP FOR BATTERY GRID APPLICATION AND ELECTROCHEMICAL CELLS UTILIZING SAME

Abstract: A laminated lead alloy strip suitable for deep charge-discharge as well as conventional lead-acid battery grid application. A thin layer of an antimonial lead alloy is laminated on such side of a lead-calcium-tin alloy strip by rollbonding. The inner layer provides good mechanical strength, while the outer layers provide the electrochemical properties needed for long cycle life in deep charge-discharge batteries. Also, methods of making the strip and use of this strip as an expanded grid material in electrochemical cells or batteries.

Description: LAMINATED LEAD ALLOY STRIP FOR BATTERY GRID APPLICATION AND ELECTROCHEMICAL CELLS UTILIZING SAME FIELD OF THE INVENTION This invention relates to laminated lead alloy composite materials for use in fabricating battery^ grids... More particularly, it relates to wrought strips of a lead- calcium-tin alloy laminated on each side with an antimonial lead alloy. The invention also relates to electrochemical cells with grids constructed from these laminated materials. BACKGROUND OF THE INVENTION The use of grids produced from expanded wrought strip material has become increasingly popular in battery manufacture. As compared with conventional grid casting, the grid expansion process nakes possible the highly automatic production of battery grids. In the grid expansion process, a lead alloy is melted and continuously cast into a slab, which is then rolled into a strip. The wrought strip is then mechanically expanded into a grid. The grid expansion process is more advantageous than the cast method because it is less labor intensive and minimizes the possibility of the occurrence of industrial pollution problems. Battery grids produced by the grid expansion method have excellent functional characteristics. They are less vulnerable to failure by grid corrosion. Grid material produced by the grid expansion method is devoid of the chemical segregation and structural inhomogeneity of cast grids. For this reason, batteries made from such material usually have a longer useful active life, especially when used as supplemental power source. grids. For this reason, batteries made from such material usually have a longer useful active life, especially when used as supplemental power source. At present, due to their strength and formability properties, lead alloys of lead , calcium and tin are generally used as the material for most grids produced by the expanded grid process. These alloys recrystallize during rolling, however, and thus lack the hardness required for use as battery grid material. Furthermore, lead-calcium-tin alloys are not suitable for batteries which undergo deep charge-discharge cycles, such as so called "traction" batteries for small electrically powered vehicles. The cycle life of the lead- calcium-tin alloys is significantly inferior to the cycle life of the antimonial lead alloys which have conventionally been used in the fabrication of cast grids. Traction batteries are typically made of cast antimonial lead due to the good past adherence of the cast material during deep charging-discharging cycles. Thus, U.S. Patent 4 ,401 ,730 discloses sealed, deep cycle lead acid batteries constructed with positive grids formed from cast antimony-lead alloys containing no more than about 2 weight percent of antimony and negative grids formed from alloys that are essentially antimony- free. Furthermore, these batteries have a highly porous, easily wetted separator material between the plates and a sufficient void volume to permit oxygen gas transport therethrough. This patent discloses the typical construction of sealed lead-acid batteries and its content is expressly incorporated by reference herein. ' These cast alloys, however, have been subject to various drawbacks and also have not been manufactured by the expanded grid technology. Various modifications to cast antimonial lead battery grids have been made to overcome problems such as antimony ion migration, contamination of the battery electrolyte, and deposition on the battery cathode. _U.S. Patent 2,952 ,726 discloses a method for reducing the content of chemically active antimony in batteries, the grids of which are fabricated of antimony- lead alloys. This method involves the reduction of the antimony content in the surface layer portions of the positive electrode plates and pasting the negative electrode plates with an active electrode mass mixed with additives which act against self discharge by binding metallic antimony deposited on the negative plates during operation. The additives consist of about 0.1 weight percent of the electrode mass of polymers of isoprene or isoprene derivatives in liquid form. The surface antimony content may be reduced either by acid treatment with concentrated sulfuric acid, electrolytic treatment to form hydrogen which reacts with antimony to form gaseous antimony hydride, or by glow discharge treatment which similarly results in hydrogen generation followed by gaseous antimony hydride formation. U.S._ Patent.3,933,5_2 discloses the use of non- antimonial lead alloys or antimonial lead alloys with less than 0.5 weight percent of antimony, onto which a coating of pure antimony has been applied to a surface density of from 0.0002 to 0.00132 gm/cm 2 . The alloys are used for the fabrication of the positive plates of a battery grid to increase the cycle life of the batteries. Such coatings may¬ be applied in a number of ways including electroplating, spraying, vapor deposition, sputtering and chemical displacement. Such solutions however cause other problems to occur in batteries so fabricated, namely, the grids show the same high charging potential as antimony-free grids under the same conditions. Also, the thin deposited coating contains porosity which detracts from its performance. Similarly, U.S. Patent . 4,107 ,.407_.discloses a grid for the positive electrode of a storage battery fabricated from a base of lead or lead alloy essentially free from antimony and coated with a lead alloy of a different compo¬ sition. The coating composition is selected such that the resulting grid achieves charging potentials of the same magnitude as are achieved with grids formed of lead alloys containing 6 to 12 weight percent antimony, while minimizing the possibility of antimony poisoning of the cathode. The surface alloy disclosed,there contains from 3 to 95 weight percent of one or more of the metals copper, silver, gold, zinc, cadmium, germanium, indium, thallium, gallium, tin, - arsenic, antimony, bismuth, selenium, tellurium, chromium, 1 molybdenum, nickel and cobalt. The methods for applying the surface alloy there include electroplating, dipping in molten alloy, spray metallization and deposition of evaporated metal in a vacuum. These methods deposit very thin layers of surface alloy, and porosity is again a source of problems in the performance of the alloy. Other references disclose the use of additional materials such as rare earth metals in lead-antimony alloys to improve the performance of battery grids fabricated with the alloy, especially with respect to grain size and corrosion. Thus U.S. Patent 4,433_,405 discloses the addition of between 0.001 and 0.1 percent of mixtures of rare earth metals, including misch metal to lead-antimony alloys containing less than 4 percent antimony. The reference also discloses the inclusion of between 0.005 and 0.1 percent copper and, optionally, arsenic and tin. Alternatively, suggestions have been made to use b different lead alloy materials in the construction of these battery grids. For example, U.S. Patent 4,125,690 discloses the use of lead-aluminum-calcium alloys alone or in conjunction with lead-tin-calcium alloys. This patent further discloses that where cold-wrought procedures are 0 used, the alloy may either be continuously cast as a slab and then preferably immediately rolled to a sheet or be additionally cooled so that it is rolled at about ambient temperature. The sheet may be rolled to reduce its 5 thickness by a reduction ratio of. from 2 to 20. ___•S.„Patent 4,279,977 discloses wrought, recrystallized lead-calcium and lead-calcium-tin alloy strips which are less than about 0.07 inch thick fpr use in the manufacture of battery grids. The alloy strips are 0 first cast and then unidirectionally rolled in several successive stages to reduce the thickness of the strip. The rolled strips undergo recrystallizatiόn at room temperature resulting in a microstructure consisting of a substantially homogeneous mixture of equiaxed and columnar grains. For 5 alloys with a low tin content, below about 0.35 weight percent tin, rolling is accomplished according to conventional rolling schedules, whereas with alloys of high tin content, over about 0.35 weight percent tin, a special rolling schedule including at least six substantially equal 0 thickness reductions must be used. The alloys of the patent are claimed to possess a stable tensile strength, both at room temperature and at temperatures up to 150° F. The strips of that invention are fast rolled , such as by a tandem mill. 5 None of these references disclose that antimonial. lead alloys can be prepared by the expanded grid technology in conjunction with calcium-tin-lead alloys to provide a battery grid which has good mechanical strength and an improved deep charge-discharge cycle life. BRIEF DESCRIPTION OF THE INVENTION This invention discloses the art of using laminated wrought strip for battery grid applications, especially for so called "traction" batteries used in electrically powered vehicles. The strips of the grid material consist of a layer of antimonial lead alloy, laminated to each side of a lead-calcium-tin strip. The laminated s.trip is produced by a roll bonding process at room temperature . SUMMARY OF THE INVENTION This invention involves a composite metallic allo strip fabricated from a center lead-calcium-tin alloy strip laminated on both sides with an outer layer of antimonial lead alloy. The composite metallic alloy strip of the invention is ideally suited for use as a battery grid material, especially, for traction batteries with deep charge-discharge cycles as employed in electrically powered vehicles. The composite metallic alloy strip is produced in either a batch or continuous process by roll bonding the individual center and two outer strips. The batch process is generally used for smaller scale, lower volume applications. The continuous process is preferred for the production of laminated strip material in large scale, high volume applications such as in the manufacture of conventional non-deep charge-discharge lead-acid automobile batteries. The roll bonding process is carried out at room temperature. This process, as utilized in the invention, serves to increase the cycle life and deep discharge g characteristics of lead-acid batteries constructed from battery grid material as disclosed herein. Although any of the commercially available lead- calcium-tin alloys can be used for the center strip, the preferred composition contains about 0.05 to about 0.15 0 percent calcium and about 0.01 to about 0.1 weight percent tin, with the balance being substantially lead. (Unless otherwise designated, all percentages in this application refer to weight percent.) Similarly, while a number of 5 antimonial lead alloys (ranging from almost 0 percent to about 10 percent antimony) may be utilized for the outer strip layer. It is difficult to roll antimonial lead alloys having greater than about 10% antimony. An especially advantageous composition is' about 0.5 to about 2 percent antimony and about 0.05 to about 0.5 percent arsenic, with 0 the balance being substantially lead. Other advantageous ranges for the components of the lead-calcium-tin alloy include the following: about 0.08 to about 0.10 percent calcium and about 0.1 to about 5 0.8 percent tin, preferably between about 0.3 to 0.7 percent tin. Minor amounts (less than about 0.1 percent) of other alloying elements such as aluminum, silicon, magnesium, etc. which do not detract from the desired strength, hardness, and corrosion resistance of these lead-calcium-tin alloys, 30 may be present. Also, other advantageous ranges for the components of the antimonial lead alloys include the following: about _b_ 0.2 to 0.5 percent arsenic and about 1.2 to 2 percent antimony and preferably about 1.6 to 2 oercent antimony. Minor amounts (less than about 0.5 percent) of other alloying elements such as cadmium, rare earth metals, aluminum, etc., which do not detract from the desired electrical properties of antimony arsenic-lead alloys, may also be present. This invention also encompasses the electrochemical cells constructed with a qrid of positive plates fabricated from the composite metallic alloy strip material described above and a separator material intimately contacting the grid and separating the plates thereof from one another. For certain applications, the negative plates of these cells may also be constructed from these composite metallic strip materials. From an economic standpoint, however, it is preferable to construct these negative plates from standard lead-calcium-tin alloys. The separator material should be at least about 70% porous, easily wettable and capable of wicking the electrolyte; The electrochemical cells of this invention also contain an electrolyte, such as sulfuric acid. The electrochemical cells of this invention reach at least about 75% depth of discharge after 2 hours and have an overall charge-discharge cycle life of at least about 200 hours, a significant and surprising improvement over batteries constructed with conventional grid materials and/or grid constructions. DETAILED DESCRIPTION OF THE INVENTION A laminated strip for use as a battery grid can be produced by either a batch or continuous process. For the batch process., individual lead-calcium-tin and antimonial lead alloy strips are prepared by first separately melting the alloys and static casting to slabs -measuring 2 inches x 28 inches x 33 inches. The lead-calcium-tin alloy slab is then rolled to a strip ranging in thickness from 1 inch down to 0.25 inches, and preferably 0.5 inch. The antimonial lead alloy slab is rolled to a strip which can be as thin as can be practicably rolled, and preferably is 0.08 inches. In preparing the strip according to the invention, it is preferred to control the final rolled thickness to 0.042", since this thickness is commonly utilized in the battery industry as a preferred expanded grid size. For other applications, the total thickness may vary widely over a range from about 0.001 to about 0.25. Strips having ' greater or lesser total thicknesses may also be utilized, if desired, withoput departing from this invention. ^It is preferred to provide outer- layers of antimonial lead alloys which are sufficiently thick to • improve the electrical properties of the grid but below that which will reduce the contribution of the lead-calcium-tin to the overall strength of the strip. Thus, when the final thickness of the inner_layer, is 0.037, the preferred final thickness for each outer layer ranges from 0.0002 to 0.04". When the inner layer is other than 0.037" thick, the preferred thickness for each outer layer will be in the jangeι of about 1:5 to 1:100 (i.e. , rjat o ^ of each outer layer to the inner layer) and most.preferably between about 1:20 toi 1:50. Alt .ough each outer layer is usually the same thickness, outer layers of different thickness can be easily provided and utilized if desired. The rolling of the strip produces a multi-layer structure wherein the outer layers are relatively porosity free and are metallurgically bonded to the inner layer. This also enables the strip to provide the improved I -10- properties when utilized as an expanded grid in an electrochemical cells. In addition, the cost of a rolled laminate is signi icantly lower in comparison to providing such outer layers by electrodeposition, coating, or otherwise depositing antimonial lead alloys on lead- calcium-tin substrates. Also, the problem of attempting to provide a specific alloy composition by electrodeposition or other coating processes is avoided by this invention. Any specific alloy is easily provided by rolling the desired composition before laminating the layer by roll bonding. The starting thickness of the individual strips can be selected to obtain the desired final thickness for each layer as well as the total thickness. For example,- to obtain a preferred grid structure, two pieces of 0.08" antimonial lead alloy are sandwiched around an inner piece of a 0.5" lead-calcium-tin alloy. As one skilled in the rolling art would realize, however, these thicknesses can be varied to obtained different final thicknesses. Thus, by selecting different layer thicknesses before rolling, different ratios of layer thicknesses can be easily obtained. J. lmi - In the continuou -s--**■ - τprmr:.io*n icWef"τsτ"'s, lead-calcium-tin alloy slabs are continuously cast, to a thickness of 3/4 inch . ' The slabs are then directed to a rolling mill where they can be cold rolled to the desired thickness. The antimonial lead slabs for use in the continuous lamination process are also initially rolled to a thinner desired thickness before laminating. This thinner gauqe is required for continuous processing so that the antimonial lead strips may be coiled easily. One or two pieces of lead-calcium-tin strip are sandwiched between two pieces of antimonial lead strip and pack rolled to a final desired thickness of the laminate. In this continuous process, two antimonial lead strips are continuously fed to the roll bonder from coils on either side of the center lead-calcium-tin alloy strip to form the laminated sandwich. The preferred finished_rolled . , strip , exhibits a ^ thickness distribution of .0.037 inch versus 0.0025 inch for the center layer of lead-calcium-tin alloy and the two outer layers of antimonial lead alloys respectively. . / /yM ~t The roll b.Q&dlxiCL.operation is accomplished using a rolling mill having a 20 inch diameter roll and capable of handling a 30 inch wide strip. Such machines are well known in the industry as "Two-Hi" rolling mills. In roll bonding, a reduct on ^ of at least about 30% and preferably J50% or more is required to achieve the appropriate bonding between the layers. Typically, an 80 to 95% reduction is utilized. Rolling ^ is performed "c_oldT_, at ambient temperature, however, the rolled strip experiences. a temperature rise due to frictional heat generated during the rolling operation and attains a maximum temperature of about 185°F at the end of the rolling operation. By following the proper roll bonding procedure, no detectable porosity in the outer layers or delamination defects can be seen at the bonding interface under microscopic examination up to 1000 X. The resulting laminated strips have a tensile strength of about 6 to 9 ksi and an elonqation of about 10-25%. " The laminate is then slit and expanded into a grid having the desired dimensions. The slitting operation is accomplished using a conventional strip slitter machine which is well known in the industry. EXAMPLES Example 1. Preparation of Comparative Battery Grids Battery tests were conducted using four types of grids, namely, (1) cast antimonial lead grid, (2) expanded antimonial lead grid, (3) expanded lead-calcium-tin alloy grid and (4) the laminated grid of the present invention. The same alloy composition was used for both the cast and expanded antimonial lead grid. The laminated grid was produced by roll bonding of antimonial lead strip and lead- calcium-tin strip. Chemical analysis of the two alloys used in the preparation of the grid tested showed the following compositions: Antimonial lead alloy 1.9% Sb - 0.32% As - 0.22% Sn - balance Pb Lead-calcium-tin alloy 0.067% Ca - 0.54% Sn - balance Pb The cast grid was produced using a conventional grid caster. For expanded grid, the alloys were melted and static cast to slabs of 2 inches x 28 inches x 33 inches. Slabs were then rolled to a finished gauge of 0.042 inch for grid expansion. For the laminated grid, the antimonial lead was rolled to 0.08 inch strip and the lead-calcium-tin alloy was rolled to 0.5 inch strip. Two pieces of lead-calcium-tin strip were then sandwiched between two pieces of antimonial lead strip and pack rolled to 0.042 inch. The finished rolled strip exhibited a thickness distribution of 0.037 inch versus 0.0025 inch for the center layer of the lead- calcium-tin alloy and the outer layers of the antimonial lead alloy, respectively. Tensile strength of this laminated strip was found to be 7.5 ksi with an elongation of 15 percent. All the 0.042 inch strip was slit and then expanded into a grid having a width of about 2.68 inches. Example 2. Comparative testing of Batteries Utilizing Prepared Grids Comparative tests of batteries constructed utilizing the four types of grids prepared according to Example 1, above, were made to determine the deep charge- discharge cycle performance characteristics of the batteries. The cells were constructed using, as far as possible, methods and procedures similar to those employed in commercial battery fabrication. The plate size of each of the batteries was approximately 6 inches x 4 inches. The grids were pasted with a conventional high density formula. Efforts were made to keep the weight of paste per unit area of positive plate constant regardless of grid type. Each test cell consisted of three plates. Commercially available separators of the type used in the manufacture of traction type batteries (such as armor-rib with 0.020 glass mat, manufactured by W.R. Grace) were used between the plates. The cells were formed conventionally using a sulfuric acid electrolyte with a specific gravity of about 1.11 and constant current. After formation, the cells were dumped and refilled with acid having a specific gravity of about 1.32. Adjustment of the specific gravity of the electrolyte in the individual cells was then made to bring the final value to within the range 1.265 to 1.275. Prior to the beginning of the automatic cycling, manual cycles were run on each cell to determine initial cell capacities. Discharges were run at rates of 1.5, 8, 10 and 14 amps. Using the capacity information obtained from the manual cycling test, automatic cycling apparatus was set to provide a 2 hour discharge followed by a 9.5 hour charge twice each day with a discharge yield of 75% depth of discharge. The cells were charged for 9.5 hours through current limiting resistors from a regulated 2.6 volt bus. The current was limited to a 2 hour rate at the beginning of the charge phase of the cycle. Following the charge phase there was a one-half hour rest period before commencement of the discharge phase. The discharge phase was carried out to yield a 75% depth of discharge after 2 hours. As the cells progressed toward the end of the discharge phase, the voltage dropped below 1.75 volts before the end of the 2 hour discharge period. When that occurred, and automatic cell cut-off circuit terminated the discharge at 1.75 volts in order to avoid an unrealistically deep discharge cycle. Cell failure was defined to have occurred when cell capacity fell below 50 percent of the capacity of the 10th cycle. Base capacity was defined as the 10th cycle value in order to avoid initial transients. Cells were cycled at room temperature, separated from each other, to minimize heating effects. Results of the tests showing the deep charge-,, discharge cycle life for all types of grids are shown in Table I. The data shown is the average of 3 cells. The results indicate that the cycle life of laminated grid is significantly better than that of lead-calcium-tin alloy grid, and better than that of both the cast and expanded antimonial lead grids. .Table I: Cell Cycle Life Grid Type Cycle Life (hours) Laminated Grid . -207 Lead-Calcium-Tin Grid 172 Cast Antimonial Lead 182 Wrought Antimonial Lead 195 Examples 3-8. Preparation of additional laminated alloy strips Various thickness of antimonial lead alloy, strips were sandwiched around different thicknesses of lead- calcium-tin alloy strips. These multi-layer strips were then rolled as described in the specification to a desired final dimension. The various thickness of the initial and final strips are illustrated below in Table II. In these strips, the antimonial-lead alloy strip comprises 2% Sb, 0.3% As, 0.2% Sn , balance Pb, while the lead-calcium-tin alloy strip comprises 0.07% Ca, 0.05% Sn, balance Pb . Table II: Laminated Strip Rolling A. Thickness (in.) before rolling into laminate Example layer 3 4 5 6 outer layer (top) 0.08 0.16 0.04 0.16 0.16 0.008 inner layer 0.5 ' 0.5 0.5 0.5 0.5 0.5 outer layer (bottom) 0.08 0.16 0.4 0.08 0.01 0.04 B. Laminate Thickness (in.) Exaπp B . layer 3 4 5 6 7 8 outer layer (top) 0.016 0.01 0.003 0.011 0.0035 0.0016 inner layer 0.1 0.032 0.04 0.034 0.011 0.1 outer layer (bottom) 0.016 0.01 0.032 0.005 0.0002 0.008 Total laminate thickness 0.132 0.052 0.075 0.05 0.0147 0.1096 10 The foregoing non-limiting examples illustrate embodiments of the invention. The invention encompasses all other examples which one skilled in the art may conceive within the scope of the claims as set forth hereinafter. 15 20 \ 25 30 35

Claims: - - THE CLAIMS What is claimed is: 1. A wrought composite strip comprising an inner layer of a lead-calcium-tin alloy laminated on each side with a outer layer of an antimonial lead -alloy. 2. The composite strip of claim 1 wherein said lead-calcium-tin alloy comprises about 0.01 to about 0.15 weight percent calcium, about 0.01 to about 1 weight percent tin, and the balance being substantially lead , and wherein said ant_imonial lead alloy comprises about 0.5 to about 2 weight percent antimony, about 0.05 to about 0.5 weight percent arsenic, and the balance being substantially lead. 3. The composite strip of one claims 1 or 2 wherein the thickness of each outer layer ranges from about 0.0002 to 0.04 inches and the thickness of the inner layer ranges from about 0.001 to about 0.25 inches. 4. The composite strip of one of claims 1, 2 or 3 wherein the ratio ^ of ^ thicknesses for each outer layer to the inner layer ranges from about 1:5 to about 1:100. 5. The composite strip of one of claims 1, 2, 3 or 4 wherein the inner and outer layers are roll bonded by pack rolling. 6. An expanded battery grid material comprising the composite strip of one of claims 1, 2, 3, 4 or 5, said strip subjected to an expanding operation to achieve the desired size and configuration. 7. An electrochemical cell comprising: a sealed container; a plurality of positive plates comprising th expanded battery grid material of claim 6; a plurality of negative plates; means to separate said positive and negative plates; and an electrolyte . 8. The electrochemical cell of claim 7 wherein the means to separate the positive and negative plates comprises a material which is at least about 70% porous and capable of wicking the electrolyte. 9. The electrochemical cell of one of claims 7 or 8 wherein the electrolyte comprises sulfuric acid. 10. The electrochemical cell of one of claims 7, 8 or 9 wherein the negative plates comprise the same expanded battery grid material as the positive plates. 11. A deep charge-discharge cycle traction battery for an electric vehicle comprising the electrochemical cell of one of claims 7, 8, 9 or 10; said battery capable of providing at least about 75% depth of discharge after 2 hours and an overall charge-discharge cycle life of at least about 200 hours.

Inventors: HAAS FRANK E, LING FU-WEN, HAAS, FRANK, E., LING, FU-WEN
Applicants: REVERE COPPER & BRASS INC, REVERE COPPER AND BRASS INCORPORATED
Application year: 1985
Main classification: H01M10
IPC codes: B32B15, H01M4