Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2000-12-07
2003-06-24
Kalafut, Stephen (Department: 1745)
Metal working
Method of mechanical manufacture
Electrical device making
C029S623100, C029S002000, C429S233000
Reexamination Certificate
active
06582482
ABSTRACT:
FIELD OF INVENTION
The present invention relates to the field of current collectors, electrodes and lead-acid batteries. More particularly, the present invention relates to current collectors composed of a lead or lead alloy substrate and a tin cladding applied to the substrate surface and batteries utilizing such current collectors, such batteries being characterized by both high cycle life and long shelf life. The present invention further relates to methods for manufacturing batteries utilizing this type of current collector.
BACKGROUND OF INVENTION
Despite considerable research into the development of improved electrochemical storage devices, the lead-acid battery remains a predominant device for delivering electrical current in many electrical operations. A conventional lead-acid battery such as the valve-regulated lead-acid (VRLA) battery is comprised of a plurality of cells. Each cell typically includes a set of interleaved monopolar positive and negative electrodes or “plates.” The electrodes typically are composed of a lead or lead-alloy current collector and an electrochemically active paste which is coated onto a surface of the current collector. The current collector typically is in the form of a grid but may also be in other forms such as a solid foil or film. The paste on the positive electrode plate contains lead dioxide when charged and is called the positive active material; the negative electrode contains a negative active material, typically sponge lead. Electrodes of opposite polarity are separated one from the other by a porous electrically insulating separator material such as a glass microfiber mat. The cell is completed by adding an acid electrolyte between the positive and negative electrodes and enclosing the entire assembly within a suitable case. A charging process activates the cell.
A major goal in the field of lead-acid batteries is to develop batteries having increased cycle life and longer shelf life. Cycle life is defined as the number of discharging and recharging cycles a battery can sustain while still delivering a certain level of electricity. Cycle life is dependent upon a number of factors including testing conditions and cell construction. With regard to testing parameters, for instance, a cell which achieves 80% of its initial amp-hour rating after 500 cycles but delivers only 50% of its initial amp-hour rating after 1,000 cycles will have two different cycle life values, depending upon whether the cell is rated at 80% or 50% of initial capacity. A related parameter, “total useable capacity,” refers to the number of cycles achieved during the cell's life multiplied by the amp-hours delivered during each cycle. It is equivalent to the area under a curve in which discharge capacity (in amp-hours) is plotted against cycle number and is also a measure of the useful work a cell can provide.
Shelf life simply refers to the usable life of a battery when it is not in use. The shelf life of batteries is affected by a process called “self-discharge,” i.e., chemical reactions within the cell which cause the consumption of electrolyte, even when the cell is not exposed to an external load. The consumption of electrolyte through self-discharge decreases discharge capacity because the discharge capacity of a cell is proportional to the specific gravity, or concentration, of electrolyte within the cell. Self-discharge not only reduces storage time and discharge capacity but also results in voltage decay, or a decrease in open circuit voltage.
Cycle life and shelf life are dependent in large measure on the chemistry which occurs at the interface between the current collector of the positive electrode and the electrochemically active paste. This interface is referred to as the “corrosion layer” or “passivation layer” depending on the conductivity of the layer. While all of the chemistry that takes place at this interface is not fully understood, battery technologists currently believe that a conductive corrosion layer (which may be a semi-conducting layer) is necessary to obtain long cycle life in lead-acid batteries. However, with certain lead and lead-alloy grids or foils, in particular pure lead, lead-calcium and lead-low tin compositions, a passivation layer (i.e., a non-conducting layer) can form. Passivation layers are composed primarily of lead oxide (PbO). The lead oxide acts as an electrical insulator and can reduce conductivity such that current cannot pass from the active material through the layer without a significant voltage loss. Thus, whether a conductive or passivation layer exists at the current collector/paste interface can dramatically impact the electrochemical properties of a cell.
In particular, the formation of a conductive corrosion layer, achieved at least in part through appropriate selection of current collector composition, beneficially results in a cell having a long cycle life. However, the drawback to a corrosion layer is that the cell generally has reduced shelf life due to the ongoing corrosion or oxidation of the lead or lead alloy current collector which consumes needed electrolyte. In contrast, current collectors whose composition tends to create passivation layers have excellent shelf life but relatively poor cycle life and recovery from deep discharge and stand. The extended shelf life is a consequence of the passivation of the corrosion layer which protects the current collector from corrosion and self-discharge and thus voltage decay; yet, as noted above, the passivation process also acts to inhibit current flow during charging, thereby reducing cycle life. Thus, cycle life and shelf life are inversely related with regard to the effect of the conductive/passivation layer. A conductive corrosion layer enhances cycle life but reduces shelf life; a passivation layer, in contrast, negatively affects cycle life but increases shelf life. Consequently, cell design involves choosing materials with the realization that a composition which enhances cycle life typically involves a tradeoff wherein shelf life is sacrificed and vice versa.
One approach to optimizing cycle life and shelf life has been to utilize lead-tin alloy current collectors in place of traditional pure lead current collectors. It has been found that the inclusion of small percentages of tin in the grid reduces the formation of a passivation layer, thereby enhancing the cycle life of a cell. It is thought that a relatively high tin content results in the tin being corroded, presumably to soluble tin(II) or insoluble SnO
2
. The corroded tin compounds are incorporated into the passivation layer where the tin compounds act as a conductor to ameliorate the insulative effects of the passivation layer, thereby enhancing conductivity and current flow between the current collector and the positive active material.
Several patents describe current collectors in which a lead-tin alloy film is superimposed on a lead or lead alloy substrate. United States patents describing this approach include U.S. Pat. No. 4,107,407 to Koch, U.S. Pat. Nos. 4,939,051 and 4,805,277 to Yasuda et al., and U.S. Pat. No. 4,761,356 to Kobayashi et al. Unlike the present invention, these patents do not describe the use of a pure tin cladding, nor do they describe current collectors wherein the tin is distributed only at the very outer surfaces of the current collectors and especially wherein the tin is nonhomogeneously distributed at the surface such that there are particles or regions of tin interspersed among high lead regions at the current collector surface.
In U.S. Pat. No. 5,024,908 (the “908 Patent”) to Terada et al., a tin-coated substrate is prepared for use in a lead acid cell. However, the 908 Patent teaches away from the current collectors of the present invention in which tin is clad to the outer surface of a substrate by stating that there are problems associated with using current collectors which are tin plated. In particular, the 908 Patent claims that during charging and formation the tin plating can disintegrate to create tin particles that can form deposits at t
Bhardwaj Ramesh C.
Gillman Leland M.
Alejandro R
GP Batteries International, Ltd.
Kalafut Stephen
Procopio Cory Hargreaves & Savitch LLP
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