Page 304 - Lindens Handbook of Batteries

13.10 PrImArY BATTErIES

prevents migrating silver ions from reaching the anode 23,24 by reducing them to insoluble silver metal.
The cellophane is oxidized and destroyed in the process, making it less effective for long-life cells.
many types of laminated membranes are presently available. A commonly used alternate barrier
material is prepared from a radiation graft of methacrylic acid onto a polyethylene membrane. 23,24
The graft makes the film wettable and permeable to the electrolyte. Studies have shown that a lower
resistance polyethylene barrier membrane is suitable for high-rate KOH cells, while higher resis-
tance polyethylene is suitable for low-rate naOH cells. Cellophane is used in conjunction with the
grafted membrane as a sacrificial barrier. The lamination of cellophane to either side of the polyeth-
ylene membrane results in a synergistic action for stopping silver migration. 15
A separator is commonly used in conjunction with a barrier membrane layer as added protection
to the barrier. It is located between the barrier and anode cavity and is multifunctional both during
cell manufacture and in performance. Separators in zinc/silver cells are typically fibrous woven or
nonwoven polymers such as polyvinyl alcohol (PVA). The fibrous nature of the separator gives it
stability and strength that protects the more fragile barrier layers from compression failure during
cell closure or through penetration of zinc particles. The separator also acts to moderate the effects of
dimensional stresses in the barrier layers developed during the lamination processes. These stresses
are relieved as the barrier membranes wet up.

13.3 CONSTRUCTION

Figure 13.9 is a cross-sectional view of a typical zinc/silver oxide button type battery. Zinc/silver
oxide button cells are designed to be anode limited; the cell has 5 to 10% more cathode capacity
than anode capacity. If the cell were cathode limited, a zinc-nickel or zinc-iron couple could form
between the anode and the cathode can, resulting in the generation of hydrogen gas.
The cathode material for zinc/silver oxide cells is monovalent silver oxide (Ag O) mixed
2
with 1 to 5% graphite to improve the electrical conductivity. The Ag O cathode material may
2
also contain manganese dioxide (mnO ) or silver nickel oxide (AgniO ) as cathode extenders. A
2
2
small amount of polytetrafluoroethylene (Teflon™) may be added to the mix as a binder and to
aid pelleting.
The anode is a high surface area, amalgamated, gelled zinc metal powder housed in a top cup,
which serves as the external negative terminal for the cell. The top cup is pressed from a triclad metal
sheet: the outer surface is a protective layer of nickel over a core of steel. The inner surface that is
in direct contact with the zinc is high-purity copper or tin.
The cathode pellet is consolidated into the positive cup, which is formed from nickel-plated steel
and serves as the positive terminal for the cell. To keep the anode and cathode separated, a barrier
disk of cellophane or a grafted polymeric membrane is placed over the consolidated cathode. The
entire system is wetted with potassium or sodium hydroxide electrolyte.
The gasket serves to seal the cell against electrolyte loss and to insulate the top and bottom cups
from contact. The gasket material is made from an elastic, electrolyte-resistant plastic such as nylon.
The seal may be improved by coating the gasket with a sealant such as polyamide or bitumen to
prevent electrolyte leakage at the seal surfaces.

Top cup
Bottom cup
Zinc anode
Sealing gasket
Silver oxide Barrier/separator
cathode

FiGURE 13.9 Cutaway view of typical zinc/silver oxide button type
battery.