Page 262 - Handbook of Battery Materials

232 8 Metallic Negatives

With primary cells the disposal of the remaining fraction of the zinc powder, or its
collection for recycling, with the used battery is not troublesome. Zinc powder is not
expensive and the rejected material has done its work as an electronically conductive
component of the anode mass. By contrast, the rechargeable (reusable) version
of the zinc/manganese dioxide cell requires – as stated before – a capacity-limiting
anode with good and reproducible (!) discharge process efficiency. For this purpose
it is necessary to establish a conductive matrix in the anode space of the cell by
admixing lightweight materials with good surface conductivity (and not taking part
in the redox reactions of the discharge and charge processes) to the anode gel.
This conductive matrix is able to prevent the formation of isolated zinc particles
out of contact with the current collector during discharge (a function of particular
importance in mercury-free cells), but also serving as a three-dimensional substrate
for the zinc deposition on charge [184].
For original equipment manufacturing (OEM) applications, that is, recharging
strings of series-connected cells mounted inside the device, the possibility of
utilizing the chemical oxygen–zinc reaction (to provide a certain overcharge
capability) has been demonstrated in a modified version of the RAM cell [185].
The efforts made to obtain a rechargeable (reusable) version of the zinc/air battery
had to overcome a series of other restrictions. This is not surprising because, in
addition to the chemical differences, this system had to deal with a scale-up to sizes
applicable to electric vehicle propulsion.
One of the first attempts was called ‘mechanical recharge.’ It was a simple exchange
of flat-plate anodes at a certain degree of oxidation (discharge) for new ones [186].
The next step was provision of the zinc electrode as a pumpable slurry as well as
on-board and central recharge schemes [187, 188].
Finally, the development of stable (‘bi-functional’) air electrodes favored the
construction of real ‘in-cell’ recharge systems [189–191] (see also Refs [177, 178]).

8.3.7.5 Zinc Electrodes for Zinc-Flow Batteries
There are three types of zinc-flow batteries (belonging in general to the group of
flow or redox batteries) which have been studied intensively: two of them are similar
with respect to the reactants involved, the zinc/chlorine [(−) Zn/HCl, ZnCl 2 /Cl 2
(+)] and the zinc/bromine [(−) Zn/HBr, ZnBr 2 /Br 2 (+)] batteries; the third one
uses an alkaline electrolyte and potassium ferricyanide as active cathode material
[(−) Zn/NaOH/K 3 [Fe(CN) 6 [(+)] [192].
The anodes of all three start with zinc provided as aqueous halide solutions
(AB2C3).
While the zinc/chlorine battery was preferred for utility load-leveling applications
[193], the zinc/bromine system was regarded as the more promising one for electric
vehicle requirements [194, 195].
Both share more or less the same merits but also the same disadvantages. The
beneficial properties are: high OCV (2.12 and 1.85 V respectively), flexibility in
design (because the active chemicals are mainly stored in tanks outside the (usually
bipolar) cell stack), no problems with zinc deposition in the charging cycle because
it works under nearly ideal conditions (perfect mass transport by electrolyte