The subject invention relates to interelectrode separators for eliminating zinc dendrite bridging between opposite polarity electrodes in secondary alkaline zinc batteries.
Battery separators function essentially to electrically isolate the positive and negative plates from each other. They prevent direct contact and suppress "treeing" or interelectrode dendrite growth which causes shorting of the respective plates. An ideal separator would isolate the plates without inhibiting electrolyte mobility, and without increasing the battery's internal resistance.
For many years, regenerated cellulose was the most commonly used separator for secondary zinc alkaline batteries (e.g., silver-zinc, nickel-zinc). In this regard, fibrous sausage casing and unplasticized cellophane (i.e., 0.025-0.5 mm dry thickness) were wrapped at least one time around the zinc electrode. Often several turns of the separator were used to insure adequate dendrite resistance. These cellulosic separator materials were generally utilized in conjunction with an "interseparator" which was a loosely woven or bibulous material wrapped or heat-sealed around the positive electrode to prevent the cellulosic separator from contacting the highly oxidizing environment of the positive electrode. Cellulosics performed well in vented, flooded cells but lacked long-term stability in sealed cells and at temperatures above the ambient (i.e., up to 100.degree. C.). Other problems with cellulosic separators involved swelling and degredation due to oxidation.
More recently, newer types of separators with better chemical stability and physical properties have become available. One such class of separators has been designated I/O inorganic/organic) separators and comprises a major inorganic component in a highly stable organic polymer binder. Such separators typically include various inorganic oxides, hydroxides, etc. in a fibrillated polytetrafluoroethylene (hereafter PTFE) binder. These materials are highly flexible and quite durable in the alkaline electrolyte of a secondary zinc battery. The process for making such materials essentially consists of the mixing of the PTFE and inorganic materials under mild shear conditions (e.g., liquid blender) with a high-boiling naphtha lubricant at temperatures above 19.degree. C. to cause fibrillation of the PTFE (see HS-10 bulletin by E. I. Du Pont de Nemours & Co., Plastics Dept., 1968). PTFE levels are normally in the 1-25% by weight range.
It is an object of the present invention to provide a new, low cost separator which is strong, flexible and particularly useful in preventing zinc dendrite growth between the electrodes of a secondary alkaline zinc battery. This and other objects and advantages of the present invention will become more readily apparent and understood from the following detailed description and illustration thereof.