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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.
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