Patent Application
20060068294
The present invention is directed to a lead acid battery that is a hybrid of gelled electrolyte lead acid battery and an absorbent porous mat lead acid battery. More particularly, the present invention is a valve-regulated lead acid battery having gelled electrolyte contained within the pores of porous glass separator mat, which are compressibly disposed between positive and negative electrode-plates.
A lead-acid battery is comprised of one or more each of positive and negative plates that are spaced apart and inter-leafed with a separator between each plate pair, together with an electrolyte constituted from diluted sulfuric acid. When a lead-acid battery is discharged, sponge lead (Pb) at the negative plate and lead dioxide (PbO2) resident at the positive plate are converted into lead sulfate (PbSO4). Thus, during discharge the sulfate of the sulfuric acid is absorbed into the electrodes. These reactions are reversed during charge, but a significant amount of secondary reactions also occur, thus requiring overcharge. The reaction at the positive electrode is:
The reaction at the negative electrode is:
The overall reaction is:
The primary overcharge reaction is the electrolysis of water that produces gaseous oxygen at the positive plate and hydrogen gas at the negative plate. In flooded (or vented) battery designs the oxygen and other gasses escape from the battery resulting in battery maintenance that requires the addition of water. In a valve-regulated lead acid (VRLA) battery the oxygen generated from the electrolysis of water is recombined with the sponge lead at the negative plate thereby suppressing hydrogen evolution, as follows:
2 Pb+O2+2 H2SO42 PbSO4+2 H2O
Valve-regulated lead acid batteries utilize a sealed cell configuration to avoid the escape of moisture and/or gases. A valve, however, is included for release of gas to avoid possible over pressurization. The VRLA battery avoids the frequent maintenance of adding water of vented or flooded lead acid batteries. A characteristic of many VRLA batteries is the immobilization of the acid electrolyte between the electrodes.
One type of VRLA battery utilizes a gelled electrolyte between the electrodes. The gelled electrolyte generally consists of a mixture of sulfuric acid and fumed silica. The gel immobilizes the free electrolyte or acid between the electrodes. The gel tends to reduce stratification of the acid or migration of the acid toward the bottom of the cell, a concern with tall flooded battery designs. Over time, however, the gel tends to dry out and shrink thereby losing contact with the electrode and adversely affecting performance. This is especially true in motive power applications where voltages and temperatures are not tightly controlled.
Another type of VRLA battery utilizes a porous mat, such as an absorbent glass mat (AGM) for the separator. The porous separator mat is typically made from a nonwoven felt of microfine glass fibers that allow the liquid electrolyte or acid to be absorbed into the pores of the mat and to be somewhat immobilized thereat. For improved contact between the electrode plates and the porous mat, the porous mat may be compressibly disposed between the electrodes. The porous mat, however, often loses compression over time thereby adversely affecting the battery performance.
A hybrid of these two types of VRLA batteries is a battery having a porous mat with liquid electrolyte absorbed into its pores and also having peripheral portions of the porous mat and/or the electrodes surrounded and sealed with a gelled electrolyte. Such encapsulation by the gelled electrolyte helps to diminish water loss from the porous mat. The encapsulation also provides for a larger electrolyte reservoir within the battery. An example of such a battery is disclosed in U.S. Pat. No. 5,376,479 to Gerner, the contents of which are incorporated herein by reference.
Different techniques for sealing peripheral portions of porous mat/electrode assemblies include sequential addition electrolyte and gel-forming components or mixtures. For example, U.S. Pat. No. 5,338,596 to Bechtold et al. describes the addition sulfuric acid without any gelling agent to fill the porous mat with liquid acid and then describes the addition of a silica/sulfuric acid gelling mixture to seal peripheral portions of the cell. U.S. Pat. No. 4,391,036 to Kishimoto et al. describes the addition of sulfuric acid with a low concentration of silica followed by a second addition of sulfuric acid with a high addition of silica to seal peripheral portions of the cell. Such peripheral sealing, however, did not address the concerns of maintaining porous mat and electrode contact over time.
Other patents describe the incorporation of a free space between electrodes and/or the porous mats in a battery design where the free space is filled with gelled electrolyte. For example, U.S. Pat. No. 4,317,872 to Varma describes a battery having its electrodes wrapped with a microfiber mat. The wrapped electrodes are described as being spaced-apart or separated by a separator. A silica/sulfuric electrolyte mixture is described as being added to the battery case, including the free space between wrapped electrodes. U.S. Pat. No. 3,457,112 to Reber describes a lead acid battery having its positive electrode plates covered by a porous mat and having gelled silica/sulfuric acid electrolyte disposed in free spaces between the wrapped positive electrode plates and unwrapped negative electrode plates. In other words, the '112 Patent describes discrete segments spaced-apart electrodes, porous mats and gelled electrolyte. U.S. Pat. No. 3,172,782 to Jache describes positive and negative electrodes being covered by porous fibrous material and being spaced-apart by corrugated spacers. A sulfuric acid/silica gelled electrolyte is described as filling the free spaces between electrodes. WO 03/067684 A2 describes the use of a never dried precipitated silica slurry and sulfuric acid as a gelling mixture. The gelling mixture is placed within the battery including free spaces between the electrodes and porous separator mats. While these patents or publications describe advantages for the use of gelled electrolyte in free spaces between battery electrodes, such gelled filled free spaces increase the overall dimensions of the battery case as compared to batteries not having such free spaces and do not fully address the concern of gel dry out. In addition, they do not address the need for maintaining a compressive force while fully immobilizing the electrolyte.
U.S. Pat. No. 1,583,445 to Collins describes a gelled electrolyte of silicic acid and sulfuric acid having glass wool particles acting as a binder to prevent cracks or fissures being formed in the gelled electrolyte. The glass wool particles may be added into the silicic acid and sulfuric acid or may be placed between the electrodes before the silicic acid and sulfuric acid solution is added to the battery. Such a gelled electrolyte would not be useful in a VRLA battery because the lack of cracks or fissures would prohibit oxygen transfer and thereby prevent or limit oxygen recombination.
Porous separator mats have been described as being formed with a solid gelling agent contained therein. For example, U.S. Pat. No. 5,605,549 to Zucker describes a porous separator pad as being a nonwoven polymeric pad having internal pockets containing powdered acrylic polymer and fumed silica particles. The pad is placed between electrodes within a battery and upon contact with acid the acrylic particles swell causing the pad to expand against the electrodes and the fumed silica forms a gelled electrolyte. U.S. Pat. No. 4,150,199 to Tuphorn describes forming a porous absorbent glass mat for use with a lead acid battery where the mat is described as being a densely compressed compact of fiber glass and fumed silica. The mat is placed between positive and negative electrode plates. Upon the introduction of acid, a gel is described as being formed within the mat. U.S. patent application Publication No. 2002/0192562 to Ferreira et al. describes the embedding of fumed silica particles into a porous mat. The mat may be placed between electrodes, and upon the introduction of sulfuric acid, a gelled electrolyte is described as being formed within the mat. GB 2,383,678 A describes a porous mat being formed from fiber glass fibers and fumed silica particles. The silica-containing mat may be compress between electrodes. Upon the introduction of the acid electrolyte, a gelled electrolyte is described as being formed in the mats. The use of porous mat that are preformed with solid gel-forming agents, however, is expensive and sometimes cumbersome as care must be taken to avoid loss or maldistribution of the powdered gelling agent in the porous mat. Presence of the siliceous materials stiffens the mat and thereby creates difficulties for battery assembly and for establishing the necessary compressive force between the electrode plates.
Despite the efforts of the prior art, there is a need for a lead acid battery having a porous separator mat which can be compressibly disposed between battery electrodes and which can have gelled electrode contained therein.
In one aspect of the present invention, a method for making a lead acid battery having gelled electrolyte disposed among the pores of an absorbent mat is provided. The method for producing the inventive lead acid battery includes the steps of:
In this aspect of the present invention, the electrode plates are desirably placed into the container prior to introduction of acid into the container. Further, the preformed positive plate comprises PbO2, and the preformed negative plate comprises lead metal, Pb, in a non-oxide form.
Desirably, the mat has an uncompressed porosity of about 95% or greater and a compressed porosity of about 90% or greater.
Useful materials for the mat include boron-silicate glass fibers, typically in a nonwoven form. It could be blended with a small quantity of polymeric fibers.
Desirably, the silica is an alkali metal polysilica, preferably, a sodium polysilica. Useful silica particle sizes range from about 4 nanometers to about 20 nanometers.
In another aspect of the present invention, a battery produced by the methods of the present invention is provided.
In another aspect of the present invention, a lead acid battery is provided. The lead acid battery includes (i) a positive electrode plate; (ii) a negative electrode plate; (iii) an absorbent glass mat compressibly disposed between the electrodes, wherein the glass mat is in substantial contact with both electrodes; and (iv) gelled electrolyte disposed within the pores of the compressed glass mat and around peripheral edges of the glass mat, wherein the gelled electrolyte comprises colloidal alkali metal polysilica having a particle size from about 4 nanometers to about 20 nanometers and sulfuric acid. Desirably, the compressed is a boron-silicate glass fiber mat and has a compressed porosity of about 90% or greater.
The porous mat 14 is desirably made from nonwoven material. The term “nonwoven” and its variants refer to fibrous materials which are generally melt blown or spun bonded (e.g., extruded onto a moving web on a conveyer belt.); prepared by a wet-laid process (e.g., where a flocculated mixture of fibers is passed through a screen/sieve with the removal of water); or prepared by a dry-laid process such as carding. Nonwoven mats may additionally be bonded by thermal, chemical or mechanical means for improved stiffness.
The fiber diameter of the fibers making the mat 14 may range from about 0.1 micron to 30 microns or more, more desirably between about 0.2 to 2.0 microns. Further, the mat 14 should have as much porosity or void volume as possible. As used herein the term porosity and it variants refer to a ratio of the total void or free volume versus the total apparent volume occupied by the mat, and is expressed as a percentage. The void volume is calculated by difference, knowing the density of the solids that comprise the mat. Desirably, the mat 14 has a porosity of about 95 percent or greater before being compressed. After being compressed, the mat 14 of the present invention still has a high porosity, for example about 90 percent porosity or greater. These porosity values, however, are not limited and mats with other porosity values may suitably be used. For example, mats with a porosity, either compressed or uncompressed, of at least 60% or greater may be used, more desirably at least 80% or greater, and preferably at least 90% or greater.
The porous mat 14 should be able to withstand a harsh chemical environment created by the presence of liquid electrolytes, such as sulfuric acid. Desirably, the mat 14 may be formed from a chemically resistant glass fiber composition, such as borosilicate glass fibers. Certain polymeric materials could replace the glass fibers or be blended with the glass fibers. Suitable polymeric materials for the mats 14 include, but are limited to, polyolefins such as polyethylene, polypropylene, polyisobutylene, and ethylene-alpha-olefin copolymers; acrylic polymers and copolymers such as polyacrylate, polymethylmethacrylate, polyethylacrylate; polyvinyl ethers such as polyvinyl methyl ether; polyacrylonitrile; polyvinyl ketones; polyvinyl aromatics such as polystyrene; polyvinyl esters, such as polyvinyl acetate; copolymers of vinyl monomers with each other and olefins, such as ethylene-methyl methacrylate copolymers, acrylonitrile-styrene copolymers, ABS resins, and ethylene-vinyl acetate copolymers; natural and synthetic rubbers, including butadiene-styrene copolymers, polyisoprene, synthetic polyisoprene, polybutadiene, butadiene-acrylonitrile copolymers, polychloroprene rubbers, polyisobutylene rubber, ethylene-propylene rubber, ethylene-propylene-diene rubbers, isobutylene-isoprene copolymers, and polyurethane rubbers; polyesters, such as polyethylene terephthalate; polycarbonates; polyimides; and polyethers. Thus, the mat 14 suitably may be a combination of glass fibers and polymeric fibers.
As depicted in
The cover 26 also contains a negative post 28, positive post 30 and a relief valve 32. Relief valve 32 is normally closed to prevent the release of moisture and/or oxygen, but may open in the advent of an over pressurization within battery 20. The negative post 28 is electrically connected to the negative electrodes 12 via a negative post strap 40. Similarly, the positive post 30 is electrically connected to the positive electrodes 16 via a positive post strap 42. The negative and positive electrodes 12, 16 have electrode support elements 18, 22, respectively, which support active metal material on the electrodes.
The present invention is not limited to any particular dimensions for the plates 12, 16. Desirably, the plates 12, 16 are thin-walled plates to minimize the overall dimensions of the battery 20. Nonlimiting examples of useful plate thicknesses include negative plates having a thickness of about one half inch or less, for example from about 0.040 inches to about 0.200 inches, and positive plates having a thickness of about one half inch or less, for example from about 0.050 inches to about 0.300 inches. The plates are lead or lead-comprising plates. Desirably, the plates 12, 16 in the assembly 10 are preformed, i.e., are in their active state, and are dry-charged into the container 24. In the preformed or active state the lead in the negative plate 12 is in the form of Pb sponge, and the lead in the positive plate 16 is in the form of PbO2. Prior to formation the lead in the plates 12, 16 are in the form of lead oxide (PbO).
A Nonlimiting method for pre-forming the lead plates of the present invention includes the steps of:
The formed plates 12, 16 are then combined with the absorbent porous separator mat 14 to complete the assembly 10. One useful, but non-limiting, method for forming the assembly 10 from preformed plates 12,16 include the steps of:
Additional details of such wrapping are disclosed in U.S. Pat. No. 6,667,130 to Misra et al., the contents of which are incorporated herein by reference.
After the compressed assembly 10 is disposed within the container 24 and after assembling the other elements of the battery 20 as illustrated in
An electrolyte mixture that is the precursor for the thixotropic gel of the present invention, which advantageously comprises an aqueous colloidal dispersion of alkali metal polysilica with sulfuric acid, is added to the sealed battery 20. To introduce the electrolyte mixture into the battery 20, the relief valve 32 is removed to reveal an opening 48 at the top of the cover 26. A nozzle (not shown) is secured to the opening 48. The nozzle has a two-way valve or port for the alternate operations of pulling a vacuum on the battery 20 and subsequently introducing the thixotropic gel. First, gas is evacuated from the battery 20 by pulling a vacuum from about 20 to about 30 inches of mercury (Hg), desirably from about 25 to about 28 inches of Hg. The electrolyte mixture (gel precursor) is then introduced into the battery 20. The moss shield 38 protects the mat 14 and electrode assembly 10 during the introduction of the electrolyte mixture. The use of a vacuum advantageously ensures its penetration into the pores of the compressed mats 14. The vacuum also removes gasses from the cell element, which also aids in the distribution of the gel throughout the pores of the compressed mat 14. The electrolyte mixture may be added in a single step or in several steps. If it is added in several steps, then a vacuum, i.e. 20 to 28 inches of Hg, is pulled on the battery 20 prior to each addition step. Desirably, the electrolyte mixture is introduced from 1 to about 5 steps, preferably about 3 steps.
An importance consideration of the present invention is the flowability of the electrolyte mixture that eventually forms the thixotropic gel within the cell, especially into the pores of the compressed mats 14, and prevention of premature gelling as the dry-charged plates 12, 16 soak up some of the acid from the gelling solution. Desirably, the thixotropic gel of the present invention comprises an aqueous colloidal dispersion of alkali metal polysilica with sulfuric acid. A useful colloidal dispersion is represented by the formula if:
[Y2O].x[SiO2].nH2O,
where Y is an alkali metal, preferably sodium, x ranges from about 20 to about 350, and n is a number of moles of water used to in the colloidal solution. Advantageously, the particle size of the colloidal silica is extremely fine at about 4 to about 20 nanometers (nm), preferably about 10 nm. The thixotropic gel is formed by combining the colloidal polysilica and sulfuric acid. Useful weight ratios of acid to polysilica include from about 2:1 to about 10:1, desirably from about 4:1 to about 6:1, preferably about 4.5:1 to about 5.5:1. Further, the specific gravity of the colloidal polysilica may vary from about 1.1 to about 1.3, preferably about 1.15 to about 1.25. The weight percent polysilica may vary from about 10% by weight to about 40% by weight, preferably about 25% by weight to about 35% by weight. Additional details of useful colloidal polysilicas and additional details of useful thixotropic gel formed therefrom are described in U.S. Pat. No. 4,889,778 to Misra et al., the contents of which is incorporated herein by reference. Useful colloidal polysilicas are commercially available as Nycaol® from Akzo Nobel. Moreover, the specific gravity of the acid may vary from about 1.2 to about 1.45, desirably from about 1.3 to about 1.4. Still further, the resulting specific gravity of the thixotropic gel may vary from about 1.25 to about 1.4, desirably from about 1.28 to about 1.35. The thixotropic gels of the present invention are typically much finer or smaller in silica particle size than mixtures made from fumed silicas. Fumed silicas often have 0.2 to 0.4 micron-sized particles, and such larger sized particles would not adequately penetrate the pores of the compressed mat 14.
VRLA batteries that have compressed AGM mats that are only filled with acid lose compression over time. The loss of compression leads to a loss in battery performance. The loss of compression is believed to be due in part to mechanical forces, such as plate expansion and shrinkage due to the electrode reactions, and the liquid acid acting as a lubricant, which allows fibers to move upon being subjected to the mechanical forces. This results in a less effective contact between the plates and the separator. The gelled electrolyte of the present invention, which is contained within the pores of the compressed mat, substantially prevents such reduction of contact. In other words, the compressed mats of the present invention with gelled electrolyte contained therein have and maintain substantial physical contact with the electrode plates over time.
In one aspect of the present invention, a method for producing a lead acid battery, such as a VRLA battery, includes the steps of:
In another aspect of the present invention, a lead acid battery, such as a VRLA, battery is provided. The battery includes:
The following non-limiting examples are intended to further illustrate the present invention.
Cells shown in
In order to determine the distribution of colloidal polysilica within the glass mat structure several inventive cells were made from cells with dry charged plates. The positive plates were 6″×19″×0.250″ and the negative plates were 6″×19″×0.180″. An inventive cell was taken apart after filling and charging. A number of sheets of glass mat were removed from electrodes (each sheet of glass was approximately sized at 6 in width×20 in length). The samples of glass mat were located in the middle as well as outside locations within the plate elements. Small rectangular samples of glass mat were cut from the top, middle and bottom portions of the glass mat, that were removed from the inventive cell. Each of the glass mat samples cut into small rectangular pieces were first weighed and then placed into a vented muffle furnace at approximately 800 C for 1 hour. At this temperature, all acid and residual moisture that had been in the sample evaporated. Additional small samples were made from glass mat samples not exposed to either acid or silica. These samples were also weighed before and after being subjected to the same treatment in the muffle furnace. The weight to the blank sample was subtracted from the weight of the sample piece of glass that had been in the inventive cell.
The results from the weight differential between the blank samples and samples from the inventive cell indicated that there was the presence of silica throughout the entire glass mat sample that had been removed from the inventive cell.
Cells were formed with the plates and mats of example 1.
Various changes and modifications can be made to the present invention. It is intended that all such changes and modifications come within the scope of the invention as set forth in the following claims.
