Page 274 - Handbook of Battery Materials

244 9 Metal Hydride Electrodes

3) If the metal atoms are not mobile (as is the case in low-temperature reactions),
only hydride phases can result in which the metal lattice is structurally very
similar to the starting intermetallic compound because the metal atoms are
essentially frozen in place. In effect the system may be considered to be pseudo
binary as the metal atoms behave as a single component.

9.3
Metal Hydride–Nickel Batteries

The half cell reactions taking place in an MH x –Ni battery may be written as
follows:
−
MH x + xOH ⇔ M + xH 2 O + xe − (9.8)
−
Ni(OOH) + H O + e ⇔ Ni(OH ) + OH − (9.9)
2
2
It is in effect a rocking-chair type battery in which hydrogen is transferred from
one electrode to the other. It is also most convenient that the voltage is essentially
the same as that in the conventional Nicad batteries. It is worthwhile noting that
the NiOOH cathode has a maximum energy density, based on Equation 9.9, of
−1
289 mAh g . This may be compared with 300–400 mAh g −1 for current MH x
electrodes and >400 mAh g −1 projected for high-capacity MH x electrodes which,
though not yet developed, are certainly conceivable.
Two types of MH electrodes, comprising the AB 5 and AB 2 classes of intermetallic
compounds, are currently of interest. The AB 5 alloys have the hexagonal CaCu 5
structure, where the A component comprises one or more rare earth elements and
B consists of Ni, or another transition metal, or a transition metal combined with
other metals. The paradigm compound of this class is LaNi 5 , which has been well
investigated because of its utility in conventional hydrogen storage applications.
Unfortunately, LaNi 5 is too costly, too unstable, and too corrosion sensitive for
use as a battery electrode. Thus commercial AB 5 electrodes use mischmetal, a low
cost combination of rare earth elements, as a substitute for La. The B 5 component
remains primarily Ni but is substituted in part with Co, Mn, Al, and so on. The
partial substitution of Ni increases thermodynamic stability of the hydride phase
[9] and corrosion resistance. Such an alloy is commonly written as MmB 5 , where
Mm represents the mischmetal component. The compositions of normal and
cerium-free mischmetal are given in Table 9.2.
The other electrode type is usually referred to as the AB 2 or Laves phase
type electrode and is discussed in Section 9.3. These electrodes are complicated,
multiphase alloys with as many as nine metal components. Alloy formulation is
primarily an empirical process where the composition is adjusted to provide one
or more hydride-forming phases in the particle bulk but which has a surface that
is presumed to be corrosion resistant because of the formation of semi-passivating
oxide layers. Unlike the AB 5 alloys there are few systematic guidelines which can
be used to predict alloy properties. Eventually AB 2 alloy electrodes may be more
attractive than AB 5 electrodes in terms of cost and energy density, but that potential
is not yet realized.