Patent Publication Number: US-2003235763-A1

Title: Grid coating process for lead acid batteries

Description:
RELATED APPLICATIONS  
     [0001] This application is related to U.S. patent application entitled “IMPROVED ELECTRODE,” assigned to common assignee of present invention, filed on Feb. 21, 2002, application Ser. No. 10/080,296, having attorney docket number DP-305,735 and hereby incorporated by reference in its entirety. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Technical Field  
       [0003] The present invention relates to processes for reducing corrosion of positive current collector grids of lead acid batteries by way of coating the grids.  
       [0004] 2. Description of the Related Art  
       [0005] The lead sulfuric acid battery is well known in the art. The lead acid battery is composed of positive and negative plates immersed in a solution of sulfuric acid and water (for example, a 5M solution of H 2 SO 4  [s.g. of 1.280]) and plate separators made of porous materials. Traditionally there are six sets of positive and negative electrode plates immersed in sulfuric acid solution, thereby making up six electrochemical cells. Each cell potential is approximately 2 volts. The individual positive and negative plates are connected in series, creating a total battery voltage of 12 volts. The positive and negative electrode plates are comprised of lead and/or lead alloy current collector grids. The active material of the positive current collector grid is a paste made from a reaction of leady oxide (partially oxidized lead powder) with sulfuric acid; the key component of the paste is electrically conductive PbO 2 . The PbO 2  is reduced to PbSO 4  when the battery is in a discharge mode.  
       [0006] It is well known that the current collector grid of the positive plate of a lead acid battery is subject to a corrosion process in which the solid lead (Pb) of the grid reacts with the water from the sulfuric acid solution and is oxidized, as illustrated by the equation that follows:  
       Pb(s)+H 2 O (l)→PbO(s)+2H + (aq)+2e −   
       [0007] The Pb of the positive grid is oxidized, creating a corrosion layer of PbO 2 , PbO, and/or PbSO 4  between the positive grid and the active material. Which corrosion layer is created is dependent upon numerous factors, such as the metal composition and morphology of the positive grid, cycling conditions, temperature, and the pH of the sulfuric acid solution. Additionally, the positive grid can react with the sulfuric acid electrolyte to form pits through pores, cracks or holes in the corrosion layer. These pits reduce the physical contact between the grid and the active material, thereby reducing the electrical conductivity of the positive plate and the overall battery.  
       [0008] Cast positive grids and expanded positive grids have different morphologies, resulting in different corrosion layers. The cast grid surface contains a randomly oriented network of grains that provide equal mechanical strength in all directions and allow the expansion of the corrosion layer without creating cracks between the positive grid and active material interface in high temperature accelerated tests. The corrosion layer for such cast grids is generally PbO 2 . Oriented expanded positive grids have an elongated grain structure that is very strong in the axial direction, but not as strong in the radial direction. (This is a result of the manufacturing process, in which the slits are made in the grid strip are stretched in a direction normal to the direction of the slits). In high temperature accelerated tests, a corrosion layer of poorly conducting PbO is formed between the expanded positive grid and the active material, thereby resulting in a loss of electrical conductivity. Because of an increase in the under-hood service temperature of vehicles and because the rate of the positive grid corrosion process increases with temperature, there is a need to reduce the corrosion layer formed between the expanded positive grid and the active material. Reduction of the insulative corrosion layer improves the contact between the grid and the active material, improving the electrical conductivity of the positive electrode plate and increasing the overall life of the battery.  
       [0009] Previous approaches dealing with positive grid corrosion have included modifying an alloy composition of the positive grid, as suggested in U.S. Pat. No. 6,114,067 issued to Knauer. Knauer suggests modifying the lead alloy with copper and silver, in addition to the conventional components of lead, calcium and tin. This solution however involves the use of additional, more costly metals, thereby increasing the complexity and cost of the grid manufacturing process.  
       [0010] Another process (although not prior art) involves electrochemically depositing PbO 2  particles on the current collector grid surface, as disclosed in patent application entitled “IMPROVED ELECTRODE,” having attorney docket number DP-305,735 referred to above. However, while this process appears to reduce positive grid corrosion, it increases processing time for lead acid batteries. Another challenge with this process is that it does not allow for flexibility as to the number of electrochemical depositions of PbO 2  coatings or the thickness of the coatings.  
       [0011] There is therefore a need for an improved process of reducing corrosion of positive current collector grids by coating the grids that minimizes or eliminates one or more of the problems set forth above.  
       SUMMARY OF THE INVENTION  
       [0012] It is an object of the present invention to provide a solution to one or more of the above mentioned problems. In one aspect of the invention, a metallic grid is provided with a wet grid coating which coats the metallic grid, the grid coating comprising a mixture of polyacrylic acid and lead dioxide (PbO 2 ) particles for the grid, wherein the grid, coated with wet grid coating, is subjected to a drying process. The wet grid coating may further include isopropyl alcohol. The wet grid coating may be applied to the metallic grid via the direct gravure method or a sprayer method. The drying process evaporates the polyacrylic acid (PAA) and the grid is coated in a dry film of a dense, smooth layer of PbO 2  particles. The wet grid coating thickness (and the thickness of the resulting dry grid coating of PbO 2  particles) may vary, dependent upon several factors, such as the size of the PbO 2  particles used in the wet grid coating and the number of times the wet grid coating is applied to the grid. One advantage of this invention is the reduction of the positive grid corrosion of the grid that has been coated with the wet grid coating and then dried. Another advantage of this invention is the increased adhesion of the battery active material to the coated grid. The wet coated grid, when dried, has a remaining film of PbO 2  particles that serve as seed crystals to encourage the formation of PbO 2  during the active material formation phase.  
       [0013] In another aspect of the invention, a method for treating a positive current collector grid is provided, the method involving the steps of coating a positive current collector grid with a mixture of PAA solution and PbO 2  particles and drying the mixture-coated grid. Another advantage of this invention is that the coating process can be adaptable to current manufacturing processes because of the speed with which it can be performed (as opposed to the alternative option of electrochemical deposition). A further advantage of using the wet grid coating technique is the flexibility with which the grid can be coated (i.e. the thicknesses can vary). One advantage of this method is it, can be adapted to a manufacturing process as it is not time consuming. Another advantage to this method is it allows flexibility to create varying thicknesses of the wet grid coating, by subjecting the grid to more than one application of the wet grid coating or by altering the composition of the grid coating, such as altering the size of the PbO 2  particles used.  
       [0014] A positive electrode plate for a lead acid battery is also provided, in which the plate includes at least one metallic grid that has been coated with a wet grid coating of a mixture of PAA and PbO 2  particles, and then subjected to a drying process, and active material covering the dried, coated grid.  
       [0015] Other features, objects, and advantages will become apparent to one of ordinary skill from the following detailed description and accompanying drawings illustrating the invention by way of example but not by way of limitation. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016]FIG. 1 is a perspective view of a battery of a type that can include the current collector grid according to the invention.  
     [0017]FIG. 2 is a perspective view of a positive electrode plate at various coating stages according to the invention.  
     [0018]FIG. 3 is a flow chart of the method according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0019] Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views, FIG. 1 illustrates a conventional lead acid battery  10 . Components of battery  10  include a positive terminal  12 , a negative terminal  14 , positive electrode plates  18 , negative electrode plates  16 , synthetic porous separators  19 , electrical connections  20  to connect positive electrode plates  18  in series, electrical connections  22  to connect negative electrode plates  16  in series, sulfuric acid solution  24 , vent cap  26 , and a case  28 .  
     [0020] The design and operation of lead acid battery  10  is well known in the art. The anodic reaction that occurs at the positive electrode plates  18  is  
     PbO 2 +4H + +SO 4   2− +2e − →PbSO 4 +2H 2 O;  
     [0021] where PbO 2  of the active material is reduced to form solid lead sulfate, having reacted with the sulfate ion from the dilute sulfuric acid.  
     [0022] The cathodic reaction that occurs at the negative electrode plates  16  is  
     Pb+2H + +SO 4   2− →PbSO 4 +2H + +2e − ;  
     [0023] where solid lead is oxidized to form solid lead sulfate, having reacted with the sulfate ion from the dilute sulfuric acid.  
     [0024] Positive plates  18  and negative plates  16  are connected in series by their individual terminals. The terminal of the end positive plate  18  connects all positive plates  18  to positive battery terminal  12 . The terminal of the end negative plate  16  connects all negative plates  16  to negative battery terminal  14 .  
     [0025] Creation of the inventive positive electrode plates  18  is illustrated in FIGS. 2A, 2B, and  2 C. Each plate  18  contains a current collector bare grid  30 , the inventive grid coating  34 , and active material  36 . FIG. 2A illustrates a current collector bare grid  30 . Grid  30  is metallic and often a lead metal alloy. Pure lead grids can not hold their shape and do not have the mechanical strength necessary to support the active material placed on grid  30 . The most widely used grid alloys fall into two categories—those that contain antimony and those that do not. Pb—Sb alloys may be used but are known to be prone to grid corrosion and to have low oxygen evolution potential and therefore are not as preferred as antimony-free alloys for grid  30 . Antimony free alloys used for grid  30  may be Pb—Sn—Ca alloys and Pb—Sn—Ca—Ag alloys, among others commercially available. Pb—Sn—Ca alloys may be more preferable than Pb—Sn—Ca—Ag grids because silver is an expensive metal and grid alloys including noble metals such as silver have poorer active material adhesion.  
     [0026] The alloy compositions may vary. As to Pb—Sn—Ca grids, preferably, the alloy includes an upper weight percent of lead (Pb Wt %) of about 99.5, with an upper Pb Wt % of about 99 desired, and an upper Pb Wt % of about 98.5 more desired. A lower Pb Wt % of about 50 can be employed, with a lower Pb Wt % of about 90 desired, and a lower Pb Wt % of about 98.3 more desired. Also, the alloy includes an upper weight percent of tin (Sn Wt %) of about 5, with an upper Sn Wt % of about 3 desired, and an upper Sn Wt % of about 2 more desired. A lower Sn Wt % of about 0.5 can be employed, with a lower Sn Wt % of about 1 desired, and a lower Sn Wt % of about 1.4 more desired. In addition, the alloy includes an upper weight percent of calcium (Ca Wt %) of about 1, with an upper Ca Wt % of about 0.5 desired, and an upper Ca Wt % of about 0.1 more desired. A lower Ca Wt % of about 0.01 can be employed, with a lower Ca Wt % of about 0.05 desired, and a lower Ca Wt % of about 0.07 more desired. The most preferred alloy composition includes 98.5 wt % Pb, 1.5 Wt % Sn, and 0.08 Wt % Ca (Pb 98.5 —Sn 1.5 —Ca 0.08 ).  
     [0027] Grid  30  is the main route for current flow. Grid  30  may be formed by a variety of methods, including casting, punching and expanding metal, all known in the art. With a casting method, molten lead alloys are poured into molds. The punching method uses a die to cut a desired shape out of a lead alloy strip. The preferred method for making grid  30  is the use of expanded metal methods which involve making slits in a metal alloy strip and then stretching (i.e. expanding) the strip such that the desired grid shape is produced. The expanded metal grid technology is more efficient than the casting method for the mass production of standard plates for use in SLI (starting-lighting-ignition) batteries. Lighter grids are produced with the expanded grid method and there is no scrap material produced, as there is with the punching method.  
     [0028] As discussed above, the alloy composition of grid  30  and the surface morphology of grid  30  determine the morphology of the corrosion layer on the grid  30  surface due to the oxidation of the Pb in grid  30 . The corrosion layer of a cast grid has a high PbO 2  content with radial cracks in the corrosion layer. The corrosion layer of an expanded grid which contains a dense, insulative layer of PbO at the interface between grid and active material, resulting in poor electrical conductivity. This PbO corrosion layer then exfoliates from grid surface, contributing to the corrosion of grid  30 . By coating expanded grid  30 , with wet grid coating  34  and subjecting it to a drying process, the corrosion process in which non-conductive PbO corrosion layer is reduced, active material  36  better adheres to grid  32 , and electrical conductivity is improved between grid  32  and active material  36 .  
     [0029]FIG. 2B illustrates a coated grid  32 , having been coated with the inventive wet grid coating  34 . Wet grid coating  34  is a “paint-like” mixture of PbO 2  particles and a polymer/solvent solution, such as liquid polyacrylic acid (PAA) solution. The mixture may also include isopropyl alcohol and may also include distilled water. The PbO 2  particles may vary in size; generally they are less than one micrometer. PbO 2  particles used may be α-PbO 2  (orthorhombic) or β-PbO 2  (tetragonal) crystals. α-PbO 2  crystals are preferred because the film that can be formed can be very high density that would reduce the grid corrosion and provide sufficient seed crystals for the formation of PbO 2  active material during the charging reaction. Other polymers may be used in the mixture in addition to or in place of PAA include polyvinylpyrrolidone (PVP), polyethylene oxide (PEO) and polystyrenesulfonic acid. The mixture may include 30-50 weight percent PbO 2  particles and 70-50 weight percent polymer/solvent solution, the mixture not to exceed 100 weight percent. In one embodiment, grid coating  34  comprises a mixture of 70 weight percent PAA solution and 30 weight percent PbO 2  particles.  
     [0030] Wet grid coating  34  may vary in thickness. This variance may arise in several ways. Increasing the concentration of PbO 2  particles in wet grid coating  34  results in wet grid coating  34  having a greater thickness. Alternatively, grid  30  may be subjected to a coating process several times. This results in several layers of wet grid coating  34 , resulting in an overall greater thickness of wet grid coating  34 .  
     [0031] Once wet grid coating  34  has been applied to grid  32 , grid  32  is subjected to a drying process in which wet grid coating  34  is dried. Dependent upon the temperature and length of time to which grid coating  34  is subjected, the PPA and/or other polymers used and other solvent materials (such as isopropyl alcohol and/or distilled water) may be essentially removed completely. A remaining dry film of PbO 2  crystals coats coated grid  32 . The thickness of the film of PbO 2  crystals may be in the range of 5 to 500 micrometers. The PbO 2  film may have a density of 9.5 to 9.8 g/ml and is relatively uniform in thickness throughout the grid.  
     [0032]FIG. 2C illustrates a completed positive electrode plate  38 , active material  36  having been coated on coated grid  32  with dry grid coating of PbO 2  crystals. The steps involved in the creation of active material  36  are known in the art and are only briefly described herein. Traditionally, active material is formed by mixing leady-oxide powders with water and sulfuric acid and forming a paste. The paste may be coated onto coated grid  32  and then coated grid  32  may be steamed to facilitate crystal growth within the paste. The paste may then be cured through a number of reactions that take place within the paste and between the paste and coated grid  32 . The curing produces a layer at the interface between coated grid  32  and paste that provides physical and electrical communication between active material and coated grid  32 . In the forming step, the paste is converted to active material, which includes the electrically conductive PbO 2 .  
     [0033]FIG. 3 illustrates an inventive method by which grid  30  may be treated. In first step  40 , positive electrode grid  30  is coated with wet grid coating  34 , creating coated grid  32 . In next step  42 , coated grid  32  is subjected to a drying process, in which grid coating  34  is dried. In step  44 , active material is pasted on coated grid  32  (which involves any or all of the creation steps briefly discussed above).  
     [0034] Wet grid coating  34  may be applied via several methods. Grid  30  may be dipped into wet grid coating  34  for approximately one second; this process however is not the most effective or efficient method for obtaining a coat of PbO 2  particles on grid  30 . A more effective method of applying grid coating  34  is by the use of a direct gravure coater. Use of a direct gravure coating process is well known in the art. Another method of applying grid coating  32  is via a sprayer method, also known in the art. Wet grid coating  34  thickness may vary, ranging from 0.1 micrometers up to and including 500 micrometers.  
     [0035] Coated grid  32  may be subjected to a drying process which can be performed at varying temperatures for varying lengths of time. Dependent upon the length of time and temperature of the drying process, varying amounts of PAA, or other polymers, and solvent are evaporated. Coated grid  32  may be fed through a heated oven. Electrical conductivity of grid  32  is improved if the PAA, other polymers, and solvents are removed completely, leaving only a dense, smooth film of substantially only PbO 2  particles. This may accomplished by increasing the drying temperature and/or length of time grid  32  is subjected to the drying process such that the PAA, other polymers, and solvents are essentially completely removed but the surface of grid  32  has not begun to melt and a film of PbO 2  particles remains on grid  32 . To achieve minimum porosity of the resulting film of PbO 2  particles, the individual PbO 2  particles should be 500 nanometers such that close particle packing can exist.  
     [0036] While the invention has been disclosed in terms of specific embodiments thereof, it is not intended to be limited thereto, but rather only to the extent set forth hereafter in the claims which follow.