Page 260 - Handbook of Battery Materials

230 8 Metallic Negatives

In rechargeable nickel/zinc and silver/zinc batteries this problem is partly
compensated for byprovisionof amassive zincreserve.The cells are cathode-limited
and the amount of anode material exceeds the theoretically required mass by a
factor between 2 and 3.
Another problem had to be solved when the zinc electrode was made reversible:
in a battery with unstirred electrolyte or an electrolyte gel, dendritic growth of the
electrolytically deposited metal takes place. The formation of dendrites cannot be
fully suppressed by the use of current collectors with large surface areas (grids,
wire fabrics). Chemical means may provide a significant contribution [49, 157, 158,
160, 164, 165]. However, by using improved separators combined in multi-layer
arrangements, the danger of short-circuiting is reduced.
Design of zinc electrodes for storage batteries always has to find a balance between
high-rate capability (this means high specific surface area [166] and good wettability
by the electrolyte [167]), corrosion protection (with opposite requirements), and
uniform zinc re-deposition upon the current collector [168–174].

8.3.7.4 Zinc Electrodes for Alkaline ‘Low-Cost’ Reusables
It was a reasonable idea to use the intensive research work in the fields of zinc,
manganese dioxide, and oxygen electrodes on one hand, and on rechargeable metal
oxide/zinc cells (the preceding section) on the other, to develop ‘rechargeable’
versions of the cells described in Section 8.3.7.2. The manganese dioxide/zinc
system (for reason of low cost) and the zinc/air system (low cost and high energy
density) were the most fascinating ones.
The specification ‘rechargeable’ is controversial: for many battery experts it
requires the possibility of – at least – some hundred to a thousand full cycles to
call a system ‘rechargeable.’ As a compromise, cells designed for 20 to about 200
cycles are designated ‘reusable’ or ‘renewable’ (e.g., RENEWAL of Rayovac, USA,
for zinc/manganese dioxide, general name RAM of former Battery Technologies
Inc. (BTI), Canada).
In the early stages of development the most significant difficulties seemed to
come from the cathode side: it took a long time to convince people of the principle
of rechargeability of manganese dioxide [175, 176] and to find suitable catalysts for
the oxygen electrode showing a sufficiently high oxidation stability in the charging
procedure [177, 178].
Soon it became evident that the zinc anode, working in both cases under
capacity-limiting conditions, causes severe troubles too [179, 180].
Whereas in the zinc/air system the anode automatically limits the discharge
(because access to oxygen from air is practically unlimited) the anode limitation
in zinc/manganese dioxide cells has another reason: K. Kordesch and co-workers
showed that the rechargeability of manganese dioxide (i.e., the number of available
cycles) depends strongly on the depth of discharge (DOD) (Figure 8.2) [181].
It is possible to design a RAM (Rechargeable/Reusable Alkaline Manganese) cell
either for high initial (first discharge) capacity (up to 1.8 Ah for AA size, close to
the alkaline primary version) and a low cycle number, or for lower initial capacity
(that means shallower discharge of MnO 2 ) but significantly higher cycle number.