Page 259 - Handbook of Battery Materials

8.3 Battery Anodes (‘Negatives’) 229

8.3.7.3 Zinc Electrodes for Alkaline Storage Batteries
Battery systems of complex design and structure using – at least for one elec-
trode – expensive materials are (for economical reasons) mainly conceived as
storage batteries. Primary (and ‘reserve’) versions of the zinc/silver oxide battery
[(−) Zn/KOH/AgO (+)] – as a first example – are only used in particular cases
where the question of cost is not crucial, for example, for marine [143–146] and
space applications [147].
The other example, called the nickel/zinc battery [(−) Zn/KOH/NiOOH (+)], has
attracted more attention in two different versions from the viewpoints of application
and cell design: one is the small cylindrical consumer cell [148, 149], the other
is the flat-plate module for electrotraction [149, 150]. A very interesting review
with an extensive collection of references was published in 1992 [151]. In 1996, an
improved bipolar construction of this battery was presented [152]. The most recent
version was described by Humble and co-authors [153]: a nickel/zinc microbattery
developed for direct installation and use in autonomous microsystems.
The problems related to the zinc electrode grew significantly with the change
to a rechargeable (reversible) system. Whereas the discharge of a zinc electrode
in a primary cell is a simple electrochemical dissolution with little concern about
the oxidation products, these may be of particular importance in a secondary
(or storage) cell. The fact of starting with zinc oxide or hydroxide instead of
metallic zinc had only minor influence (AB2C2). In any case, the solubility of zinc
oxide or hydroxide in the KOH electrolyte was found to be a key parameter in a
reversible zinc electrode [154]. The result of zinc migration (‘shape change’) was
obvious when the electrodes of a flat-plate battery were inspected after a series of
charge–discharge cycles. The active material was removed from the electrode edges
and agglomerated toward the plate center. If the number of cycles was sufficiently
high, the edge areas of the current collector were completely denuded of zinc.
Usually, this phenomenon limits the lifetime of a battery because the storage
capacity falls under a reasonable lower limit. One reason for this zinc migration
was identified by McBreen [155]: inhomogeneous current distribution makes the
zinc move away from high current density areas. Another mechanism seems to
be active as well: electrolyte convection induced by electro-osmosis through the
separator [156]. Many attempts have been made to prevent or at least retard shape
change, preferably by reducing the solubility of zincate in the electrolyte [157–163].
The consequences of shape change are densification and loss of electrode
porosity, increased current density caused by loss of zinc surface area, and finally
earlier passivation.
Two different forms of passivation can stop the discharge of a zinc electrode
before the active material is exhausted. ‘Spontaneous’ passivation occurs at high
current densities within a few seconds. ‘Long-term’ passivation may be observed
−2
after hours of continuous discharge in a current density range of 15–35 mA cm .
The effects are explained by the existence of supersaturated solutions of ZnO in
KOH, which are normally quite stable, but if precipitation is induced by any means
(nucleation) solid products form immediately and block the electrodes.