Lead-acid battery positive plate and alloy therefore

Abstract

A lead-acid battery grid made from a lead-based alloy containing tin, calcium, bismuth and copper and characterized by enhanced mechanical properties, corrosion resistance, less battery gassing, lower sulfation and water loss, and no post-casting treatment requirements for age hardening. In one embodiment, the battery grids are formed from a lead-based alloy including about 2.0% tin, about 0.0125% copper, about 0.065% calcium, and about 0.032% bismuth. Preferably, the battery grid is free of silver beyond trace levels in the alloy.

Claims

1. A lead-acid cell having a positive plate and a negative plate disposed with a container, a separator disposed within said container and separating said positive and negative plates, and an electrolyte within said container, said positive plate comprising a grid supporting structure having a layer of active material thereon, said grid supporting structure comprising a lead-based alloy consisting essentially of lead, from about 1.5% to about 3.0% tin, from about 0.01% to about 0.02% copper, from about 0.015% to about 0.04% bismuth, and from 0% to about 0.08% calcium, the percentages being based upon the total weight of said lead-based alloy.

2. The lead-acid cell of claim 1 wherein the tin content of said alloy is about 1.75% to about 2.25%.

3. The lead-acid cell of claim 1 wherein the tin content of said alloy is about 1.95% to about 2.05%.

4. The lead-acid cell of claim 1 wherein the tin content of said alloy is about 2.0%.

5. The lead-acid cell of claim 1 wherein the copper content of said alloy is about 0.0125%.

6. The lead-acid cell of claim 1 wherein the bismuth content of said alloy is about 0.015% to about 0.035%.

7. The lead-acid cell of claim 1 wherein the bismuth content of said alloy is about 0.032%.

8. The lead-acid cell of claim 1 wherein said alloy is free of barium beyond trace levels in the alloy.

9. The lead-acid cell of claim 1 wherein the calcium content of said alloy is about 0.05% to about 0.07%.

10. The lead-acid cell of claim 1 wherein the calcium content of said alloy is about 0.065%.

11. The lead-acid cell of claim 1 wherein said alloy is free of silver beyond trace levels in the alloy.

12. The lead-acid cell of claim 1 wherein said grid is a cast grid.

13. The lead-acid cell of claim 1 wherein the battery is sealed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Referring to the FIGURES wherein like elements are numbered alike in the several FIGURES:

(2) FIG. 1 illustrates an exemplary lead-acid battery;

(3) FIG. 2 illustrates an exemplary VRLA cell;

(4) FIG. 3 illustrates main effect plots for corrosion rate from an 8-month test cell;

(5) FIG. 4 illustrates main effect plots for sample length growth from an 8-month test cell;

(6) FIG. 5 illustrates a graph of hardness evolution for Alloy A and Alloy 15;

(7) FIG. 6 illustrates a graph of yield strength evolution for Alloy A and Alloy 15;

(8) FIG. 7 illustrates a graph of tensile strength evolution for Alloy A and Alloy 15;

(9) FIG. 8 illustrates a graph of elongation evolution for Alloy A and Alloy 15;

(10) FIG. 9 illustrates a graph of polarization overvoltage for oxygen evolution on Alloys 15, A and B;

(11) FIG. 10 illustrates a graph of stand loss, or average voltage drop (mV) after 112 days for Alloy 15, 13 and A;

(12) FIG. 11 illustrates a graph of BCI cycle life capacity discharge for Alloy 15 cells;

(13) FIG. 12 illustrates a graph of low rate cycle life, C/8, 100% DoD cycle test (Alloy 15, residual capacity=95% after 580 cycles) for a 2-Volt AGM cell version; and

(14) FIG. 13 illustrates a graph of low rate cycle life, C/8, 100% DoD cycle test (Alloy A, residual capacity=80% after 600 cycles) for another 2-Volt AGM cell version.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

(15) As required, exemplary embodiments of the present invention are disclosed. The various embodiments are meant to be non-limiting examples of various ways of implementing the invention and it will be understood that the invention may be embodied in alternative forms. The present invention will be described more fully hereinafter with reference to the accompanying drawings in which like numerals represent like elements throughout the several figures, and in which exemplary embodiments are shown. The figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular elements, while related elements may have been eliminated to prevent obscuring novel aspects. The specific structural and functional details disclosed herein should not be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention.

(16) Although an industrial VRLA battery is shown in FIGS. 1 and 2, the various embodiments of the present invention may include any type of lead acid battery including, for example, transportation batteries. FIG. 1 illustrates an exemplary lead-acid battery having a positive plate, indicated generally at 10, with a separator 12 enveloping the positive plate 10. The positive plate 10 generally comprises a grid 14 having a plate lug 16 and positive active material 18 pasted onto grid 14. As is known, there are many different configurations for the grid. Additionally, in VRLA cells, the separator is typically an absorbent glass fiber mat. Other commercially available glass fiber separators incorporate polyolefin or other polymeric fibers to replace part of the glass fibers.

(17) FIG. 2 illustrates a VRLA cell, indicated generally at 20. The cell 20 thus includes a container or jar 22 retaining snugly therein an element stack, shown generally at 24. The element stack 24 thus comprises a series of positive plates 10 and negative plates 26 alternately disposed and having separators 12 separating adjacent positive and negative plates. Band 28 is used to hold adjacent plates in the desired compression and to facilitate assembly (the band encircling the element stack 24, but being partially broken away in FIG. 2 for illustrative purposes). The VRLA cell 20 likewise includes a positive terminal 30, a negative terminal 32, and a cover 34 affixed to container or jar 22 by any appropriate means, as is known. Inasmuch as VRLA cells function by oxygen recombination, as is known, a low pressure, self-resealing valve 36 is used to maintain the desired internal pressure within the cell. Many suitable relief valves are known and used.

(18) The grid 14 includes a PbSnCa type alloy with some other elements and characteristics as described hereinafter below. In one embodiment, Ca is fixed at about 0.065%, just below the peritectic composition, and preferably no Ag is added.

(19) A full factorial design of experiment (DOE) with four factors, Sn, Cu, Bi and Ba, generates the alloy matrix for lab testing. Tables 1 and 2 identify the factor levels and the alloy matrix. Alloy A is the control.

(20) TABLE-US-00001 TABLE 1 Factors and levels Factor Levels Sn 1.5% 2.0% 3.0% Cu 0.000% 0.0125% NA Bi 0.015% 0.032% Ba 0.000% 0.0175%

(21) TABLE-US-00002 TABLE 2 Alloy Matrix Alloy # Sn % Cu % Bi % Ba % 1 1.5 0.0000 0.015 0.0000 2 1.5 0.0000 0.015 0.0175 3 1.5 0.0000 0.032 0.0000 4 1.5 0.0000 0.032 0.0175 5 1.5 0.0125 0.015 0.0000 6 1.5 0.0125 0.015 0.0175 7 1.5 0.0125 0.032 0.0000 8 1.5 0.0125 0.032 0.0175 9 2.0 0.0000 0.015 0.0000 10 2.0 0.0000 0.015 0.0175 11 2.0 0.0000 0.032 0.0000 12 2.0 0.0000 0.032 0.0175 13 2.0 0.0125 0.015 0.0000 14 2.0 0.0125 0.015 0.0175 15 2.0 0.0125 0.032 0.0000 16 2.0 0.0125 0.032 0.0175 17 3.0 0.0000 0.015 0.0000 18 3.0 0.0000 0.015 0.0175 19 3.0 0.0000 0.032 0.0000 20 3.0 0.0000 0.032 0.0175 21 3.0 0.0125 0.015 0.0000 22 3.0 0.0125 0.015 0.0175 23 3.0 0.0125 0.032 0.0000 24 3.0 0.0125 0.032 0.0175 25 Alloy A 26 Alloy B (pure lead)

(22) Dog bone samples were gravity cast. Hardness and tensile tests were performed to evaluate the mechanical properties of the alloys as-cast and aged at the room temperature and heat treated at 100 C. for three hours.

(23) The corrosion tests were performed at 60 C., in H.sub.2SO.sub.4 with 1.30 s.g., and with potential of about 1.30V vs. Hg/Hg.sub.2SO.sub.4 reference electrode. Four samples of each alloy were used for each test cell. The test time is from 1.5 to 11 months. After the corrosion test, the weight loss and sample dimension changes were measured and the corrosion layer was analyzed with a scanning electron microscope (SEM) and optical microscope.

(24) For the 1.5, 5 and 8-month corrosion test cells, the effect of variables were similar. FIGS. 3 and 4 illustrate the main effect plots for the corrosion rate and sample length growth from the 8-month test cell. Both Sn and Bi reduce the corrosion rate and sample growth. Cu almost has no effect on the corrosion rate but reduces the sample growth. Ba had a negative effect on the corrosion rate and sample growth.

(25) The effect of Sn on the corrosion rate is more significant from 1.5 to 2.0% than from 2.0 to 3.0%. Therefore, the preferred alloy is with 2.0% Sn, 0.032% Bi and 0.0125% Cu (Alloy 15 in the matrix) and was selected for battery build tests.

(26) FIGS. 5-8 compare mechanical properties for Alloy A and Alloy 15. With higher Ca content, Alloy 15 has higher hardness (FIG. 5), yield (FIG. 6) and tensile (FIG. 7) strength with the similar elongation (FIG. 8).

(27) Oxygen overvoltage measurements were carried out in 1.30 SG sulfuric acid at 25 C., 35 C., and 45 C., respectively, at electrodes fabricated from Alloys 15, A, and B (pure lead). The Tafel parameters derived from the Tafel equation (overvoltage)=a+b Log [i] are presented in Table 3, below, where a is the intercept and b is the slope. b represents the overvoltage per a decade increment in current density, and a is related to the exchange current density at the open circuit voltage by the relationship Log [i.sub.o]=a/b. The Tafel slope, b, is the same for all the 3 test alloys. The Tafel slope is related to the reaction mechanism for oxygen evolution at the test electrode. What this means is that the mechanism for oxygen evolution appears to be independent of alloy type or the operating temperature.

(28) TABLE-US-00003 TABLE 3 Tafel Parameters for Alloys 15, A, and B in 1.30 Acid Tafel Parameters i.sub.0, mA/cm.sup.2 Sample (25 C.) (35 C.) (45 C.) ID Alloy Composition b (mV) Ea (kJ/mole) 10.sup.6 10.sup.6 10.sup.6 Alloy 15 Pb0.065Ca2.0Sn0.012Cu0.032Bi 110 0.2 56.9 3.9 0.9 1.1 1.6 Alloy A Pb0.04Ca0.025Ag2.0Sn 111 0.2 36.6 4.2 2.0 2.4 4.1 Alloy B Primary pure Pb 110 0.1 52.5 3.7 1.0 1.7 2.7

(29) The exchange current density, io, describes the rate of reaction (for O.sub.2 evolution) under open circuit conditions. The higher the value, the greater is the rate of O.sub.2 evolution at that electrode. A higher exchange current density would signify a catalytic effect of the alloy on oxygen evolution. The exchange current density for alloy 15 is appreciably lower than that for Alloy A. In fact, it is comparable to that of Alloy B (pure Pb). Furthermore, the activation energy for oxygen evolution on alloy 15 (similar to that of Alloy B) is significantly higher than that of Alloy A.

(30) A plot of the polarization voltage at a specific current density of 10 mA/cm2 in FIG. 9 confirms a much higher polarization resistance for Oxygen evolution on Alloy 15 than on Alloy A. Consequently, the rate of oxygen evolution on Alloy 15 would be expected to be relatively lower than that on Alloy A but comparable to that of Alloy B (pure Pb). The lower gassing rate could lead to lower self-discharge rates in the battery and to a more efficient recombination process in VRLA systems.

(31) Bismuth in combination with copper and tin in the alloy raises the oxygen overvoltage. Alloying additives that raise the oxygen overvoltage have the propensity to mitigate the impact of gassing at the positive plate. Alloy 15 is comparable to Alloy B (pure lead) in terms of resistance to gassing at the positive plate.

(32) Cell Test Results

(33) Based on the above lab test results, Alloy 15 (Pb2.0Sn0.065Ca0.032Bi0.0125Cu) is selected for the battery test. Test results are illustrated in FIGS. 10-13.

(34) FIG. 10 illustrates stand loss, or average voltage drop (mV) after 112 days for Alloy 15, 13 and A.

(35) FIG. 11 illustrates BCI cycle life capacity discharge for Alloy 15 cells.

(36) FIG. 12 illustrates low rate cycle life, C/8, 100% DoD cycle test (Alloy 15, residual capacity=95% after 580 cycles) for a 2-Volt AGM cell version.

(37) FIG. 13 illustrates low rate cycle life, C/8, 100% DoD cycle test (Alloy A, residual capacity=80% after 600 cycles) for another 2-Volt AGM cell version.

(38) As can be seen, the up-to-date cell test results show that alloy 15 performs equally or better than Alloy A. Other alloys, including but not limited to, alloys 13, 14 and 16 may adequately satisfy the diverse requirements needed for making battery grids for positive plates. Moreover, these alloys are characterized by enhanced mechanical properties, corrosion resistance, less battery gassing and water loss, enhanced electrical performance, and no post-casting treatment requirements for age hardening so that the grids can be processed much sooner after being cast. These criteria should be satisfied regardless of the type of application.

(39) It is important to note that the construction and arrangement of the elements of the alloy for a battery grid as shown in the preferred and other exemplary embodiments is illustrative only. Although only a few embodiments of the present inventions have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g. variations in combinations and subcombinations of the amounts of the alloy elements) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. For example, elements may be substituted and added, and the amounts of the elements may vary. Accordingly, all such modifications are intended to be included within the scope of the present invention as defined in the appended claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the preferred and other exemplary embodiments without departing from the spirit of the present inventions as expressed in the appended claims.