Page 257 - Handbook of Battery Materials

8.3 Battery Anodes (‘Negatives’) 227

a basket which is automatically inserted and prevents direct contact of the anode
material with the cathode and the bottom of the cell container.
Anodic active species and electrolyte are provided as a gel consisting of zinc
−1
powder, aqueous KOH solution (7–9 mol L ), gelling agents, and additives. Finally
the current collector (a brass nail spot-welded to the metallic part of the cell top) is
introduced when the cell top is placed and the can is crimped to give a gas-tight
closure.
Cells of cylindrical geometry are produced mainly in four sizes: D (LR-20),
C (LR-14), AA (LR-6), and AAA (LR-03).
The two other alkaline cells of this section (using HgO or an oxygen electrode
as cathode) were almost exclusively produced as small button cells. Larger zinc/air
batteries gained some attention as candidates for electric vehicle propulsion [124].
The change from zinc sheet to zinc powder improved the high-current perfor-
mance of the cell significantly but it increased the corrosion problems (a larger
specific surface means a higher corrosion rate). Consequently, evaluation of the per-
formance of zinc alloy powders in gelled electrolyte [125], the search for particularly
active (high-rate) zinc anodes [126], and for methods to fabricate porous zinc anodes
[127] were initiated. The electro-dissolution of zinc in alkaline solutions [128] and
zinc corrosion (gassing) after partial discharge [129] have been thoroughly studied.
The discharge reactions now include formation of hydroxo complexes, preferably:

Zn 2+ + 4OH → [Zn(OH) ] 2− (8.22)
−
4
Depending on electrolyte saturation and KOH concentration, subsequent precipi-
tation reactions may follow:
[Zn(OH) ] 2− → Zn(OH) + 2OH , or: (8.23)
−
4 2
[Zn(OH) ] 2− → ZnO ↓+ 2OH + H 2 O. (8.24)
−
4
The equilibrium concentration of zincate [130] as well as zinc oxide morphology and
distribution in discharged Zn/MnO 2 batteries [131] became matters of particular
interest.
In competition with the electrochemical discharge reaction and consequently
diminishing the shelf life of the cell, chemical dissolution (corrosion) of zinc is
more or less active:
Zn + 2OH + 2H 2 O → [Zn(OH) ] 2− + H 2 ↑ . (8.25)
−
4
The loss of active zinc and the evolution of hydrogen, causing an intolerable rise of
internal cell pressure, were retarded by amalgamation of zinc particles. For quite a
long time (up to about 1982) mercury contents of 6% (in some cases up to 8%) were
regarded as normal [132]. Then a rapid decrease in the mercury content took place
(Figure 8.1). The first step was a reduction of Hg to 3%, which was made possible
by the application of new amalgamation methods (surface-amalgamation instead
of or additional to volume amalgamation [133, 134]. This first step was followed by a
further decrease in the mercury content to 1% when the amalgamation techniques
were regarded as suitable for achieving this goal [135]. In the meantime it was
found that extremely pure zinc powder (made from selected raw material by a gas