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13.18        PrImArY BATTErIES

                             solubility of the additives placed in the cathode, and the degree to which they will promote parasitic
                             reactions in the anode if they dissolve into the electrolyte over time.


                 13.8  CONSTRUCTION

                             Button and coin cells are the most prevalent form for zinc/air batteries to take. Development of other
                             formats, such as cylindrical and prismatic forms, has been somewhat limited due to economics, but
                             also by the frustrations of air management and the poor performance of zinc/air when use is intermit-
                             tent. moisture sensitivity in large cells becomes a greater problem when rate capability demands a
                             largely open cathode to air interface. Unless the cathode can be isolated from the environment during
                             the idle periods, the cells will suffer the adverse effects of water gain or loss, as well as the degrada-
                             tion caused by the accumulation of carbonates at the air-electrolyte interface.
                                Similar in many ways to their predecessors, the zinc/mercury oxide and zinc/silver oxide button
                             cells, the zinc/air button cell is designed with an eye to ultimate electrical capacity within a defined
                             volume. As illustrated in Fig. 13.16, the zinc/air button cell can pack more zinc in the same volume
                             because the system doesn’t require a thick metal oxide cathode to complete the cell reaction. Each
                             cell size has an analogue size in the two older chemistries, with the exception of the size 10 cell (IEC
                             Pr70), which was developed by rayovac in the mid-1980s. It was introduced to the hearing instrument
                             industry as an enabling technology for in the ear (ITE) and completely in the canal (CIC) devices.
                                As  seen  Fig.  13.17  there  are  no  spare  parts  in  the  construction  of  the  zinc/air  cell.  Each  part
                             performs multiple functions and must do so while occupying a minimum of volume of its own. The
                             resulting assembly contains the maximum amount of zinc that can be discharged by the user, deliver-
                             ing the longest number of hours or days of battery performance. The negative top of the cell is an
                             impervious container for the zinc, acts as the current collector, and resists the pressure of closure that
                             seals it against the gasket. The top also makes the electrical contact that brings the electronic current
                             out of the cell to an external device or circuit. The positive can acts as the outer container of the cell,
                             connecting electrically to the cathode disk, and applies the compressive forces of closure against the
                             gasket that stands between the can and the top. Vent holes in the can admit air to the cathode, and the
                             outer surface provides electrical contact to the external circuit. Finally, the gasket between the can and
                             the top provides a seal against electrolyte leakage and prevents electrical shorting between the top and
                             the can. The gasket is usually made of a hard polymeric material, often an engineering plastic such as
                             polyamide, and may be coated with additional sealant material to enhance its resistance to leakage.
                                Within the cell container, the cathode rests flat (see Fig. 13.18), occupying the inside bottom of
                             the can. It is a multilayer structure consisting of a gas diffusion membrane. 31,32  The first layer is a
                             hydrophobic, microporous membrane that is permeable to gases such as oxygen and water vapor, but


                                       Metal oxide battery      Top             Zinc/air battery


                                                              Grommet

                                                             Zinc anode

                                                              Separator


                                                              Cathode

                                                                Can
                                  FiGURE 13.16  Cross-sections of metal oxide and zinc/air button batteries.
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