Separators play a crucial role in alkaline batteries. They keep the positive and negative sides of the battery separate while letting certain ions go through and blocking others. The separator is a passive element that has to perform the same task unchanged for the life of the battery. Additionally, the separator must be able to withstand a strongly alkaline environment at ambient and elevated temperatures and be able to resist oxidative attacks.
To meet the high current demands of modern electronics, there is an increasing need for an alkaline battery that contains a separator that can conduct hydroxyl ions at an increasingly rapid rate. Films of cellulose in the form of regenerated cellulose have been used since World War II as the separators of choice for this purpose because of their superior ability to conduct hydroxyl ions in strongly alkaline media. Their low electrical resistance of 10 milliohm-in2 has also led to their widespread use among manufactures of zinc-based type batteries. These types of batteries include, among others, silver-zinc, zinc-nickel, and zinc manganese dioxide based batteries. Additionally, the cellulose acts as a physical barrier to migration of other ions into the battery, particularly, zincate ions and silver ions in a silver-zinc battery.
In the presence of a silver cathode, regenerated cellulose performs a sacrificial role as all of the active centers are oxidized in the presence of silver ions with the simultaneous deposition of silver metal. This can have a deleterious effect on the water transport as well as the ionic conductivity of the membrane.
Limited developments have occurred in the improvement of regenerated cellulose as a battery separator. Regenerated cellulose films are the result of extensive processing of cellulose and involve a xanthation reaction thereof. Cellophane, manufactured by regeneration from cellulose, has a degree of polymerization between 350 and 500. However, the regenerated cellulose separators presently used in zinc alkaline batteries do not have sufficient mechanical strength to withstand penetration by zinc dendrites and are subject to oxidation.
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