technical report
The Future of the Silver-Zinc Battery
- Alexandria, VA USA
To paraphrase a famous saying, the premature death of the silver-zinc battery has been exaggerated. It is a system still kicking and living in many applications where no other battery system could be used. Although the lithium-ion battery system is making good progress, its substitution for large silver-zinc batteries is not seen to materialize in the short term.
While a large lithium-ion battery has still to show all the safety aspects required for use in military applications, the silver-zinc battery is on the verge of making a radical change from the current designs where the zinc electrode and the separator material are the primary causes of failure and short life. Upon repeated cycling the zinc electrode degrades very fast and, under certain uncontrolled conditions, developed zinc dendrites pierce the separator and cause a short in the cell, thus a premature failure. Moreover, the separator used, being the traditional cellophane, degrades in the concentrated potassium hydroxide electrolyte whether the cell is used or not, thus limiting the calendar life to approximately two years.
The conditions for improvement of the cycling life and calendar life therefore reside in obtaining a radical change in the three weak points of the system, viz., the zinc electrode, the separator and electrode. The zinc electrode can be made in such a way to withstand the shape change inherent in the traditional zinc electrode. This can be done in conjunction with a low concentration of potassium hydroxide where the zinc oxide dissolution is minimal. At the same time a separator can be found to resist the low concentration of electrolyte, something the cellophane cannot do since its degradation is faster with lower concentration of electrolyte.
These conditions have been obtained to a certain extent in the nickel-zinc system that uses the same zinc electrode and the low electrolyte concentration. Some companies have been able to achieve a remarkably long cycle-life, between 500 and 1000 cycles, with a different type of zinc electrode. On the other hand, a new non-cellulosic separator material has been found to achieve a longer cycle-life (over 100%) in ordinary silver-zinc cells, using ordinary traditional zinc electrodes, cellophane, and high electrolyte concentration.
A combination of all these features can make a silver-zinc cell radically different from the traditional one, with a tremendous increase in cycle-life and calendar life. The new zinc electrode will have less tendency to dissolve and create zinc dendrites. The low electrolyte concentration would not dissolve much zinc oxide and would not attack the new separator material.
Although the system may lead to somewhat higher internal resistance, there are several applications where only low rates are used. Even if high rates are needed, the cell can be designed accordingly to accommodate them, possibly at some expense of energy densities, but a trade-off with longer cycle life and calendar life may be warranted.
If the nickel-zinc cell can give 600-1000 cycles, it is reasonable to expect that the improved silver-zinc cell may yield at least 300-400 cycles but not more, given the fact that there is still a silver penetration failure mode to contend with &emdash; a problem the nickel-zinc does not have.
Such concepts are being investigated, not so much at the research or even development stage but already at the engineering level to be applied to practical cells in the short term. Because of the proprietary nature of these elements, it is too premature to get into more details at this time. Even if the lithium-ion system is successfully realized, its high cost may be an obstacle compared to that of the improved silver-zinc system, which may have been given a new lease on life before completely dying.
Mr. Himy has written four books on silver-zinc batteries based on 26 years of working with all types and sizes of them, from button cells to submarine cells.