Page 299 - Lindens Handbook of Batteries
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BUTTOn CELL BATTErIES: SILVEr OxIDE–ZInC AnD ZInC-AIr SYSTEmS 13.5
1.8
A
1.7
1.6
B, C & D
1.5 E
Volume, V 1.4
1.3
C DEB A
1.2
1.1
1.0
10 20 30 40 50 60
Capacity (mAh) at 100k ohms continuous, 21°C
FiGURE 13.3 Comparative performances of various zinc/silver oxide chemistries,
type 392 button cell, 7.8 × 3.6 mm. (A) Zn/AgO; (B) Zn/“double-treatment” AgO;
(C) Zn/Ag O; (D) Zn/AgO-silver plumbate; (E) Zn/AgniO . Discharge on 100 kohms,
2
2
21°C.
discharge curve. The first occurs at 1.8 V, corresponding to the reduction of divalent silver oxide to
monovalent
+
2AgOH O + 2 2 → e AgO + 2 2OH - E° =+ 0 607 V
.
As the discharge continues, the voltage drops to 1.6 V, corresponding to the reduction of monovalent
silver oxide to silver metal
+
+
Ag OH O 2 → + e 2 Ag 2 OH - E° + 0 342 V
.
2
2
The overall electrochemical reaction of the Zn/AgO cell is
+
+
Zn AgO → AgZnO
This two-step discharge is not desirable for many electronic applications where tight voltage regula-
tion is required.
The elimination of the two-step discharge has been resolved by several methods. 11,14–16 One com-
mercial approach, shown schematically in Fig. 13.4, was to treat a compressed pellet of AgO with
a mild reducing agent such as methanol. The treatment forms a thin outer layer of Ag O around a
2
core of AgO. The treated pellet is consolidated into a can and then reacted with a stronger reduc-
ing agent such as hydrazine. The hydrazine reduces a thin layer of silver metal across the pellet’s
exposed surface. The cathode produced by this process has only silver metal and Ag O in contact
2
with the cathode terminal. The layer of Ag O masks the higher potential of the AgO, while the thin,
2
Methanol treated Consolidated into Hydrazine treated
AgO pellet (Ag 2 O coating) cathode can (Ag surface)
FiGURE 13.4 The Ditronic “double-treatment” process for divalent silver oxide.