TO IMPROVE ITS CYCLE LIFE

Related Application

[0001] This application claims benefit of U.S. Provisional Patent Application No. 61/327,512, filed April 23, 2010.

Copyright Notice

[0002] © 2011 Amtek Research International, LLC. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR § 1 .71 (d).

Technical Field

[0003] This disclosure relates to battery separators for use in a lead-acid battery and, in particular, to a battery separator in which, throughout its porous structure, a benzaldehyde derivative is dispersed as a hydrogen-evolution inhibitor to improve the cycle life of a deep cycle battery.

Background Information

[0004] The recombinant cell and the flooded cell are two different types of commercially available lead acid battery designs. Both types include adjacent positive and negative electrodes that are separated from each other by a porous battery separator. The porous separator prevents the adjacent electrodes from coming into physical contact and provides space for an electrolyte to reside. Such separators are formed of materials that are sufficiently porous to permit the electrolyte to reside in the pores of the separator material, thereby permitting ionic current flow between adjacent positive and negative plates. [0005] The recombinant battery, also referred to as a valve regulated lead-acid ("VRLA") battery, typically includes an absorptive glass mat (AGM) separator composed, in whole or in part, of microglass fibers. AGM separators provide high porosity and uniform electrolyte distribution with a certain fraction of the pores left open to facilitate the transport of oxygen from the positive electrode, where the oxygen is produced during charging, to the negative electrode, where the oxygen is recombined with hydrogen ions to produce water. AGM separators also exhibit low puncture resistance, which can be disadvantageous to the operation of the VRLA battery in certain applications, such as high vibration environments.

[0006] In the flooded cell battery, only a small portion of the electrolyte is absorbed into the separator. Flooded cell battery separators typically include porous derivatives of cellulose, polyvinyl chloride, organic rubber, and polyolefins. More specifically, microporous polyethylene separators are commonly used because of their ultrafine pore size, which inhibits dendritic growth while providing low electrical resistance, good oxidation resistance, and excellent flexibility.

[0007] Thus most flooded lead-acid batteries include polyethylene separators. The term "polyethylene separator" is something of a misnomer because these microporous separators require large amounts of precipitated silica to be sufficiently acid wettable. The volume fraction of precipitated silica and its distribution in the separator generally control its electrical properties, while the volume fraction and orientation of polyethylene in the separator generally control its mechanical properties.

[0008] Commercially available precipitated silica is typically combined with a polyolefin, a process oil, and various minor ingredients to form a separator mixture that is extruded at an elevated temperature through a slot die to form an oil-filled sheet. The oil-filled sheet is calendered to its desired thickness and profile, and the majority of the process oil is extracted. The sheet is dried to form a microporous polyolefin separator and is slit into an appropriate width for a specific battery design.

[0009] During battery manufacture, the separator is fed to a machine that forms "envelopes" by cutting the separator material and sealing its edges such that an electrode can be inserted to form an electrode package. The electrode packages are stacked such that the separator acts as a physical spacer and an electronic insulator between positive and negative electrodes. An electrolyte is then introduced into the assembled battery to facilitate ionic conduction within the battery. [0010] The primary purposes of the polyolefin contained in the separator are to (1 ) provide mechanical integrity to the polymer matrix so that the separator can be enveloped at high speeds and (2) to prevent grid wire puncture during battery assembly or operation. Thus, the hydrophobic polyolefin preferably has a molecular weight that provides sufficient molecular chain entanglement to form a microporous web with high puncture resistance. The primary purpose of the hydrophilic silica is to increase the acid wettability of the separator web, thereby lowering the electrical resistivity of the separator. In the absence of silica, the sulfuric acid would not wet the hydrophobic web and ion transport would not occur, resulting in an inoperative battery.

[0011] Controlled chemical reactions between active chemicals produce the desired conversion of chemical energy into electrical energy occurring in a lead-acid battery. The desired chemical reactions on which battery operation depends are also accompanied by adverse chemical side-reactions that consume or impede the reactions of some of the active chemicals. Decomposition of water and changes in the volume and composition of the electrolyte are undesirable effects of adverse chemical side-reactions occurring in lead-acid battery operation. One chemical side- reaction, hydrogen gas evolution, is greatly increased by the presence of antimony (Sb) in the grids of the positive battery electrode plates. Antimony from the grid of the positive electrode dissolves slowly into the acid electrolyte. Once in solution, the antimony diffuses throughout the electrolyte, where some of the antimony

encounters the negative electrode. Since the negative electrode is at an

electrochemical potential that is below the plating potential for antimony, the antimony becomes reduced and deposits onto the negative electrode. The deposited antimony has a lower overpotential for hydrogen evolution (i.e., hydrogen is more easily evolved from it) than that of the lead electrode. This results in an increase in hydrogen evolution during charging of the battery, a reduction in charging efficiency, and an increase in the rate of water loss from the battery. Suppression of hydrogen gas evolution would, therefore, prolong the nominal operational

performance of a lead-acid battery.

[0012] It is therefore desirable to cost-effectively produce a porous separator having a material composition that suppresses hydrogen evolution in a lead-acid battery. Summary of the Disclosure

[0013] A battery separator includes dispersed throughout its porous structure a benzaldehyde derivative as a hydrogen-evolution inhibitor to improve the cycle life of a lead-acid battery containing the battery separator. The disclosed battery separator is particularly useful in a deep cycle battery installed in an electric vehicle, such as a golf car or a floor scrubber. Preferred embodiments of the disclosed battery separator are based on a microporous polyethylene separator material that includes a microporous polyolefin web exhibiting high-strength mechanical and low electrical resistance properties. The microporous polyolefin web has dispersed throughout its porous structure vanillin (4- hydroxy 3-methoxybenzaldehyde) compound as a preferred derivative of benzaldehyde that interacts with antimony present in the battery electrode plates to suppress hydrogen gas evolution. The presence of vanillin dispersed throughout the porous structure exhibits strong antimony- suppression behavior and thereby maintains hydrogen evolution inhibitor properties during handling and manipulation of the battery separator. Other compounds exhibiting strong antimony-suppression behavior include ortho-anisaldehyde

(2-methoxybenzaldehyde), 2-hydroxybenzaldehyde, 4-methoxybenzaldehyde, 2,4-dimethoxybenzaldehyde, 2,5-dimethoxybenzaldehyde, veratraldehyde

(3,4-dimethoxybenzaldehyde), and 2,3,4 trimethoxybenzaldehyde. An alternative preferred embodiment of the disclosed battery separator is based on an AGM battery separator material also having a porous structure through which a benzaldehyde hydrogen gas evolution inhibitor of the above-described type is dispersed.

[0014] Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings.

Brief Description of the Drawings

[0015] Fig. 1 is a bar graph showing antimony selectivity ratios for different sets of battery separators.

[0016] Figs. 2, 3, and 4 are graphs showing voltammetry scan results for microporous polyethylene battery separators dip-coated in solutions of vanillin in acetone.

[0017] Fig. 5 is a graph showing voltammetry scan results for a microporous polyethylene battery separator treated in solution of vanillin in trichloroethylene (TCE). [0018] Fig. 6 is a graph showing voltammetry scan results for a microporous polyethylene battery separator treated in solution of vanillin in water.

[0019] Fig. 7 is a graph showing voltammetry scan results for a commercial deep discharge separator.

[0020] Fig. 8 is a graph showing voltammetry scan results for an absorptive glass mat (AGM) battery separator treated in solution of vanillin in acetone.

[0021] Fig. 9 is a diagram of a lead-acid cell, illustrating the position and function of the disclosed battery separator.

Detailed Description of Preferred Embodiments

[0022] First preferred embodiments of the disclosed battery separator are based on a RhinoHide® microporous polyethylene battery separator material, which is manufactured by Entek International LLC, Lebanon, Oregon. A benzaldehyde hydrogen gas evolution inhibitor is dispersed throughout the porous structure of the RhinoHide® separator. Vanillin is a preferred derivative of benzaldehyde because it is soluble in water and readily available. Distribution of vanillin throughout the porous structure of the RhinoHide® separator makes it more durable when compared to a vanillin surface coating, which would crystallize on the separator surface and fall off with handling.

[0023] The vanillin compound interacts with the antimony present at the surface of the negative electrodes of the battery to inhibit hydrogen gas evolution. Vanillin exhibits significant antimony selectivity, which is defined for a given cathode voltage, as a ratio of relative change in the hydrogen evolution current to relative change in lead (Pb) discharge capacity. A higher selectivity ratio indicates better hydrogen evolution inhibition.

[0024] The vanillin compound is applied to the RhinoHide® separator by dip coating sheets of the RhinoHide® separator material in a 1 %-3% aqueous vanillin solution heated to 50°C-75°C. Dip coating the separator material in the heated solution achieves wetting of the pore structure and good dispersion of vanillin throughout the porous structure of the RhinoHide® separator. Use of the disclosed vanillin-treated RhinoHide® separator in a deep cycle lead-acid battery installed in an electric vehicle increases the number of charge-to-discharge cycles

characterizing the cycle life of the battery.

[0025] A study was performed to measure the antimony-suppression behavior of leachates made from each of multiple trial separators based on RhinoHyde® microporous polyethylene battery separator material and produced for deep discharge golf car battery builds. The following describes the preparation and testing of the trial separators and summarizes the results obtained and conclusions drawn from tests performed.

Polyethylene Separator Material Examples

[0026] A first set of samples of separator material was dip-coated in solutions of vanillin in acetone at 0.24% and 2.4% concentrations. Batteries made with these separators demonstrated good performance compared to a Daramic, Inc., HD separator, which functioned as a control. A second set of samples of separator material included separators treated with vanillin using three different solvents:

acetone, trichloroethylene (TCE), and water.

[0027] The Antimony Suppression Test performed to carry out this study uses Linear Scanning Voltammetry (LSV) to examine the hydrogen evolving behavior of the negative lead electrode in the presence of antimony together with a candidate antimony control additive (ACA).

[0028] The leachates were prepared by adding 100 ml of 1 .210 s.g., pre- electrolyzed sulfuric acid to the following separator materials and cooking them for 4 days at 70°C:

• 2.4% vanillin-acetone (first set sample), 8.28 grams

• 0.24% vanillin-acetone (first set sample), 7.80 grams

• 2.4% vanillin-acetone (second set sample), 7.94 grams

• 2.5% vanillin-TCE, 7.82 grams

• 3.0% vanillin-water, 7.75 grams

• Daramic HD, 7.90 grams.

[0029] The Antimony Suppression Test was performed using a three-electrode cell apparatus. After three cathodic cycles between -700 mV and -1700 mV to condition the electrode, a blank LSV scan was run. The electrode was held at a fixed potential of -1200 mV for 15 minutes, then swept from -1200 mV to -700 mV at 5 mV/sec. Following the blank scan, 1 .0 ml of 0.1 % Sb ³⁺ ion in HN0 ₃ acidified solution was added to the electrolyte, resulting in a concentration of 8.2 ppm, and the LSV scan was repeated. A quantity of the leachate from the separator was then added to the test cell, and a third sweep was performed. [0030] The results of the Antimony Suppression Test are presented as antimony selectivity factors for each test run of all separators tested and are summarized in Table 1 and presented for comparison in bar graph form in Fig. 1 .

Table 1 : Selectivity ratios for all separators tested

The voltammetry scans produced for each test are shown in Figs. 2-7. The selectivity ratio for each test was determined from the relative change in hydrogen evolution current at -1200 mv divided by the relative change in Pb discharge capacity, which is expressed as:

S = dsh added / lleachate added) .

(Qsb added / Qleachate added)

Fig. 2 presents the Antimony Selectivity Test results for a separator treated with 2.4% vanillin in acetone in the first set. The curves show that suppression of the antimony behavior is significant. Fig. 3 presents the Antimony Selectivity Test results for a separator treated with 0.24% vanillin in acetone in the first set. The curves show that suppression of the antimony behavior is small but significant.

Fig. 4 presents the Antimony Selectivity Test results for a separator treated with 2.4% vanillin in acetone in the second set. The curves show that suppression of the antimony behavior is significant. Fig. 5 presents the Antimony Selectivity Test results for a separator treated with 2.5% vanillin in TCE in the second set. The curves show that suppression of the antimony behavior is significant. Fig. 6 presents the Antimony Selectivity Test results for a separator treated with 3.0% vanillin in water in the second set. The curves show that suppression of the antimony behavior is significant. Fig. 7 presents the Antimony Selectivity Test results for a Daramic HD separator, which is a commercial separator for deep discharge batteries. The curves show that suppression of the antimony behavior is significant. [0031] These results lead to conclusions that all of the leachates made from vanillin-treated separators demonstrated significant antimony suppression activity. The lower result for the 0.24% vanillin-acetone separator from the first set indicates that there is a concentration dependency to the effect. The second set of separators prepared with acetone, TCE, and water-based solutions of vanillin all demonstrate strong activity compared to the Daramic HD control.

[0032] The separator described above is delivered in roll form to lead-acid battery manufacturers where the separator is fashioned into "envelopes." An electrode can then be inserted into a separator to form an electrode package. The electrode packages are stacked so that the separator acts as a physical spacer and as an electrical insulator between positive and negative electrodes.

[0033] A second preferred embodiment of the disclosed battery separator is based on an absorptive glass mat (AGM) battery separator material, such as that manufactured by Hollingsworth and Vose, East Walpole, MA. A benzaldehyde hydrogen gas evolution inhibitor is dispersed throughout the porous structure of the AGM separator. Vanillin is a preferred derivative of benzaldehyde because it is soluble in water and readily available. Distribution of vanillin throughout the porous structure of the AGM separator makes it more durable when compared to a vanillin surface coating, which would crystallize on the separator surface and fall off with handling.

AGM Separator Material Example

[0034] A sheet of AGM separator, with a basis weight of 225 grams per square meter, was cut into a piece measuring 28 cm x 38 cm. A 1 % solution by weight of vanillin (Alfa Aesar, 99%) in acetone (Chem Products, reagent grade) was prepared. This solution was poured over the AGM sheet in a glass pan until the sheet was covered. The saturated sheet was lifted out of the solution and placed in another glass pan and allowed to dry at ambient conditions for two hours, followed by oven drying for 20 minutes at 70°C. The uptake of vanillin solution by the AGM sheet was approximately 130 ml. After drying, a portion of the vanillin-treated sheet, weighing 6.87 grams, was cut up into pieces and placed in a flask for preparation of an acid leachate. A 200 ml quantity of sulfuric acid with a specific gravity of 1 .210 was also added to the flask containing the treated AGM. The flask was then placed in an oven at 60°C for 3 days, after which the leachate was ready for testing. [0035] A study was performed to measure the antimony-suppression behavior of the leachate made from the AGM, in the manner described above. An Antimony Selectivity Test was conducted using a 5 ml portion of the leachate made from the vanillin-treated AGM. Fig. 8 presents the linear scanning voltammetry scan plots resulting from this test. The curves show that suppression of the antimony behavior is significant. The antimony suppression factor derived from the test was 1 .10, which indicates a moderate effect for suppression of the antimony behavior on the lead electrode.

[0036] Fig. 9 is a diagram of a lead-acid cell 100, which includes two electrodes 102, each with one end dipped in an electrolytic fluid or gel 104, typically sulfuric acid, and each with the other end connected by a wire 106 to an external electric circuit 108. Each electrode 102 separately undergoes one-half of an electrochemical oxidation-reduction reaction to either produce or consume free electric charge. A lead anode 1 10, or negative electrode, is oxidized in a reaction that supplies electrons 1 12. A lead oxide cathode 1 14, or positive electrode, is reduced in a reaction that consumes electrons. A main requirement is that electrodes 102 be kept separate from each other so that electron transfer is forced to occur through wire 106 in external electric circuit 108. A separator 1 16 is therefore used to divide cell 100 into a left compartment 1 18a and a right compartment 1 18b. Separator 1 16 prevents electrodes 102 from coming into physical contact with each other and short-circuiting cell 100. Separator 1 16 permits electrolyte 104 to reside in the pores of the separator material and thereby facilitates diffusion of ions 120 between left compartment 1 18a and right compartment 118b. If separator 1 16 is insufficiently porous, ionic current flow through electrolyte 104 is hindered and the electrochemical reaction may be hindered or ultimately arrested. Battery separator 1 16, based on microporous polyethylene material of the first preferred embodiments or on AGM material of the second preferred embodiment and through the porous structure of which a benzaldehyde hydrogen gas evolution inhibitor is dispersed, improves the cycle life of lead-acid battery 100.

[0037] It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.