Page 300 - Lindens Handbook of Batteries

13.6 PrImArY BATTErIES

taBle 13.2 Effect of the Coating Thickness for the “Double-Treatment” method of Divalent Silver Oxide
Voltage level with storage
Ag O thickness on Ag thickness on Final cathode capacity,
2
pellet, (mm) consolidation, (mm) (mAh/g) 1 month 3 months 6 months
0.2 0.12 372 1.73 1.77 1.80
0.6 0.12 360 1.61 1.63 1.71
1.0 0.12 326 1.60 1.59 1.59
0.2 0.24 360 1.60 1.59 1.59
0.6 0.24 348 1.60 1.59 1.59
1.0 0.24 315 1.60 1.59 1.59

conductive silver layer reduces the cell impedance. In use, only the monovalent silver oxide volt-
age is observed yet the cell delivers the greater capacity of the divalent silver oxide. Even with this
double surface treatment, the cells delivered, with the same weight of silver, 20 to 40% more hours
of service than the standard monovalent silver oxide cells.
™
Cells produced by this “double-treatment” process are termed Ditronic cells. Figure 13.4 shows
the benefit of the Ditronic design in a button cell. The treated cell has about a 30% capacity advan-
tage over a conventional Ag O cathode at the same operating voltage.
2
This “double-treatment” method has the disadvantage that the control of the treatment process is
critical to the shelf life of the cell. reducing the outer surface of the divalent silver oxide pellet to
either only monovalent silver oxide or to only silver metal does not have the advantages of the dual
process. The same is true if either coating is not sufficiently thick (Table 13.2).
During storage, the cells eventually exhibited the phenomenon referred to as “voltage up” and
“impedance up” (Fig. 13.5). The divalent silver oxide slowly oxidizes the conductive silver layer
back to the resistive monovalent silver oxide.
+
Ag AgO → AgO
2
As the metallic silver layer is depleted, the cell demonstrates an increase in open-circuit voltage
and impedance. The high impedance, due to high internal resistance, results in a low closed-circuit
voltage and a nonfunctional cell.

2.0
100 Uncoated AgO
Cell open-circuit voltage, V 1.8 Silver-coated AgO Cell impedance, ohms 80 Silver-coated AgO
90
1.9
Uncoated AgO
70
60
50
Ag 2 O-coated AgO
1.7
30
1.6 Voltage-up ↑ 40 Ag 2 O-coated AgO
20
Ditronic 10 Impedance-up ↑
Ditronic
1.5
3 6 9 3 6 9
Months at 20°C Months at 20°C
FiGURE 13.5 “Voltage-up” and “impedance-up” of Zn/AgO battery during 1 year storage at
21°C, type 392 button cell, 7.8 × 3.6 mm.