Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer or spacer insulating structure
Reexamination Certificate
2000-02-25
2002-04-16
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer or spacer insulating structure
C429S129000
Reexamination Certificate
active
06372379
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to separators for use in silver-zinc batteries, and more particularly to separators made of micro-porous membranes.
2. Description Relative to the Prior Art
Silver zinc batteries have the highest power among the alkaline batteries. They are composed of at least one pair of electrodes of opposite polarity, usually a series of adjacent electrodes of alternating polarity, positive silver electrode and a negative zinc electrode, and KOH as electrolyte. Separators are positioned in the cell between the adjacent electrodes to prevent shorting between electrodes from metal migration. The inclusion of these separators is well known in battery technology, and has been the subject of a number of patents. The separators are porous, allowing the migration of electrolyte through the separator, but preventing migration of metal particles.
Silver zinc batteries have an energy density per volume 2.5 times greeter than those of lead acid batteries and 1.8 times higher than those of nickel cadmium. The energy density per weight of a silver zinc battery is 3.7 times higher than a lead acid battery and 2.0 times higher than a nickel cadmium battery. In addition there are less environmental concerns with silver zinc batteries than with lead acid or nickel cadmium and silver-zinc batteries are safer than lithium batteries.
However, silver zinc batteries have significantly shorter cycle life than the other batteries. They have a high capacity loss and short cycle life and higher labor assembly costs than other batteries. As a result, silver zinc batteries have, up to now, been limited in their applications to those areas where high power is required, such as the military.
Cellophane is the primary choice for separators in the vast majority of both military and commercial secondary silver zinc batteries. It has been found, however, that the Cellophane separator is the primary cause of the short comings of silver zinc batteries. This Cellophane separator has been the major obstacle for producing an enhanced silver zinc battery with cycle life, calendar life and performance comparable to alkaline batteries.
In prior art silver zinc cells, either the positive or the negative electrode is commonly wrapped with 5 to 8 layers of Cellophane. An electrolyte absorbing layer is usually positioned between the separator and the electrode. Pellon (nylon) or a surface treated non-woven polyolefine mat is usually used as absorbing layer in silver battery systems.
In prior art the Cellophane acts as a “sacrificing” layer between the electrodes of silver zinc batteries. In effect, the Cellophane layer is progressively consumed as it performs its function. Cellophane is not stable in KOH or silver oxide, a powerful oxidizing agent present in silver battery systems, which attacks and oxidizes the Cellophane. Water also attacks Cellophane and makes it swell. In order to reduce the Cellophane degradation in a KOH solution a higher concentration must be used, 45% KOH is common. In a low KOH concentration (i.e., high water content) Cellophane swells and degrades rapidly. At room temperature, the electrical resistance of a 45% KOH solution is significantly higher than a 31% KOH solution. The performance of a battery with 45% KOH is, therefore, poorer than a battery that uses 31 % KOH solution as electrolyte, assuming all other parameters stay the same.
Thus, the cycle life and calendar life of today's silver zinc battery is very short due to usage of Cellophane as the main separator. The cycle life of a silver zinc battery is primarily limited to the number of sacrificing Cellophane layers used in a given cell. However, this number is limited to eight layers because Cellophane does not increase cycle life if more than eight layers are added. Eight layers of Cellophane can only withstand a maximum wet life of 2-3 years in a 45% KOH solution and designing a cell with more Cellophane layers will not increase the calendar life and it will only reduce the energy density. The calendar life of a cell using Cellophane is even shorter in a 31% KOH solution.
Celgard®
1
is used as an alternative to Cellophane as a separator for some alkaline battery applications. Celgard is a polyolefine membrane produced by sheet extrusion and gradual stretching of the sheet to produce a porous membrane. The pore diameters of Celgard are large, 1000-2000 angstrom range, (U.S. Pat. Nos. # 3,558,764 and # 5,667,911 ) and not uniform, the pore diameters in machine direction are significantly larger than cross machine direction. Although the application of Celgard is described in the prior art, Celgard is not suitable for silver zinc battery application due to its large pore size and low pore tortuosity. The cycle life of a silver zinc battery with Celgard separator is significantly shorter than the one with Cellophane. This is due to a larger pore diameter and faster rate of colloidal silver migration through the pores causing a very short cycle life, therefore, Celgard is not used for any commercially available silver zinc batteries.
The silver zinc battery separator is an insulator which must be resistant to degradation in strong alkali (such as potassium hydroxide) and heat. Further, the separator must be highly porous to allow migration of electrolyte and to provide a battery of high energy density. Another criterion is that the separator must exhibit low electrical resistance. A still further criterion is that a separator must inhibit migration of metal particles in electrolyte solution. In addition, the separator must be capable of inhibiting formation and growth of dendrites which can bridge the thickness of the separator after a period of time and cause shorting between electrodes of different polarity.
One of the other problems associated with silver zinc secondary batteries is capacity loss after each cycling which is due to a phenomena called zinc shape change. During each charge and discharge zinc oxide dissolves in electrolyte and re-plates back on the electrode. However, the zinc oxide does not re-plate necessarily at the same location. Zinc oxide usually dissolves from the top of the negative electrode and re-plates on the bottom, reducing the surface of active material. This phenomena will manifest itself by a gradual capacity loss.
Various prior art has attempted to remedy these problems. U.S. Pat. No. 5,336,573 discloses a method of making battery separators with high tensile strength by utilizing prior art technology for producing microporous membranes (extruding a mixture comprised of polymer, particulate filler and processing plasticizer to form a sheet, then calendar the sheet and extract the plasticizer in another step). instead of using a sheet die, they used a cross-head die and encapsulated a non-woven substrate inside two layers of microporous sheets. In a cross-head die, the processing mixture makes a 90 degree turn and splits to provide two feeds (one upper and one lower). At the same time, the fibrous sheet (non-woven) is fed into the die through a separate mandrel and is positioned between two feeds within the die. The two extruded feeds and the fibrous sheet meet close to the die's exit. In this region the mixture from the feeds recombines while encapsulating the fibrous sheet within its core. The main objective of U.S. Pat. No. 5,336,573 is to produce a microporous sheet with a very high tensile strength.
The separator produced, using the method of this patent had large pores, due to the use of the non-woven, macroporous web encapsulated with a microporous membrane, since the pore diameter is a function of the fiber diameter which is usually in the range of 5 microns. Therefore, the separator described here cannot be used to minimize silver migration.
In U.S. Pat. No. 4,371,596 Sheibley describes producing a flexible porous battery separator for an alkaline cell by coating a woven or non-woven substrate with a slurry comprised of a copolymer or rubber-based resin (a binder), a polar organic plasticizer (must react with
Samii Abbas M.
Samii Garrin
Brouillette Gabrielle
White Mark P.
Wills M.
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