Improved silver/zinc secondary cells for underwater applications

Performance improvements in silver/zinc technology have been achieved with new developments in additives and separator coatings. These improvements, aimed at increasing the cycle life and energy density of silver/zinc cells, were realized through advances in the stateof the-art of the zinc electrode and of the separators, which are the major limitations to this electrochemical couple. To achieve these objectives, two basic approaches were employed: (i) the introduction of a new class of materials known as electro permeable membranes (EPM); (ii) the use of bismuth oxide (Bi2O3) as an additive to the negative electrode. The EPMs are applied to the separator to provide a barrier to silver penetration without impairing performance. This allows the use of very thin separators and/or a reduction in the number of layers required. This provides an increase in the available volume for active materials with a consequent improvement in volumetric energy density. The effect of the additives to the negative electrode is to inhibit shape change and increase the conductivity of the zinc. The general approach utilized in this program was to incorporate the performance enhancing improvements into a baseline cell design. The baseline has been configured to fit two underwater vehicles currently in use by the US Navy. The development approaches and specific cell configurations are presented along with current performance data. An analysis and critique of the data coupled with conclusions and recommendations for future development activities are also presented.