Page 252 - Handbook of Battery Materials

222 8 Metallic Negatives

The electrochemical equivalent of about 480 Ah kg −1 is one of the lowest for
all metallic anodes, and the OCV of 1.35 V for the ‘Nicad’ is not favorable for
many applications. Studies of failure mechanisms [26] revealed that the cadmium
electrode is responsible for capacity loss and memory effect of the nickel/cadmium
battery. Additionally, it is desirable to restrict the use of cadmium for environmental
reasons. The consequence is a continuous retreat of this system from many
applications, and battery packs for electric tools may eventually be the only
remaining use.
The replacement of nickel/cadmium batteries by nickel/metal hydride cells may
be seen in this light. The better performance of the latter (about 30%) is a further
strong argument to pay the only slightly higher price. Additional advantages are
the flat discharge curve and the extremely good cycle life.
Recycling of valuable materials from used ‘Nicad’ batteries is an issue of growing
importance [27].

8.3.3
Iron (Fe)

Probably the best-known battery system using an iron anode is the nickel/iron
battery. It should be written: (−) Fe/KOH/NiO(OH) (+), and has its merits as a
heavy duty accumulator [28]. By far less famous and much more recent are the
applications of iron anodes in (rechargeable) iron/air cells [(−) Fe/KOH/O 2 (+)]
[29, 30] and in iron/silver oxide batteries [(−) Fe/KOH(+LiOH)/AgO (+)] [31, 32].
The composition of an iron anode includes Fe 3 O 4 (produced by partial reduction
of Fe 2 O 3 with hydrogen), iron powder, and additives (e.g., sulfur, FeS, HgO). One
group of inventors claims 2000 cycles for an iron electrode containing ZnS as
a main additive [33], others describe additive systems containing FeS (to retard
passivation of the iron electrode) and ammonium sulfate (to provide porosity) [34],
FeS and PbS to retard self-discharge [35], FeS and potassium sulfide to suppress
hydrogen evolution and to improve cyclability [36], or potassium sulfide together
with bismuth sulfide for the same reasons [37]. Relatively new is the addition of
carbon nanoparticles to increase conductivity of the discharged electrode [38, 39].
Another method of electrode precursor preparation is filling carbon nanotubes with
iron nitrate followed by decomposition of this compound in argon atmosphere
yielding pure Fe 2 O 3 [40].
The precursor mixture is converted to the active iron anode either by internal
reduction (AB2C2) or by high-temperature external reduction (AB2C1) [41, 42].
The discharge/charge of this electrode is done in two steps, but only the first step
−
(Fe ↔ Fe 2+ + 2e ) is of practical use. For the iron/nickel oxide-hydroxide system
these steps (or voltage plateaus) may be written as:

Fe + 2NiO(OH) + 2H 2 O ←→ 2Ni(OH) + Fe(OH) (8.6)
2 2
3Fe(OH) + 2NiO(OH) ←→ 2Ni(OH) + Fe 3 O 4 + 2H 2 O (8.7)
2 2