Page 284 - Handbook of Battery Materials

254 9 Metal Hydride Electrodes

3600
−
n = e = 7 (mw)(Q max ) (9.12)
9.65 × 10
−1
where mw is the molecular weight of the alloy and the units of Q are mAh g . They
assumed that after activation the remaining uncorroded alloy in each subsequent
charge–discharge cycle is hydrided and dehydrided to the same degree and n is
constant. The percent lattice expansion of the unit cell in each electrochemical cycle
was calculated via the equation
V V H
% = n × 100 (9.13)
V V
3
where V is the actual volume change of the unit cell in ˚ A in each charge or
discharge cycle, V is the initial unit cell volume, and n is the number of H atoms
inserted into the unit cell and subsequently discharged.
Finally, the loss of electrochemical capacity is directly proportional to the loss of
the AB 5 alloy by oxidation and readily calculated as follows;
%wtloss −dQ
= (Q max ) × 100 (9.14)
cycle cycle
The effects of Ce, Co, Al, and Mn upon the properties and performances of
A(NiCoMnAl) 5 electrodes employing the above equations are discussed in the
following sections.

9.3.4.1 Effect of Cerium
The rare earth composition of commercial electrodes is also related to electrode
corrosion. This was noted by Sakai et al. [44], who found that the presence of Nd
and Ce inhibited corrosion when substituted in part for La in La 1−x Z x (NiCoAl) 5
(Z = Ce or Nd) electrodes. However no explanation for the effect was noted. Willems
[23] prepared an electrode having the composition of La .8 Nd .2 Ni 2.5 Co 2.4 Si .1 , which
retained 88% of its storage capacity after 400 cycles. He attributed its long cycle life
3
to a low V H of 2.6 ˚ A .
The case of cerium is of particular interest. Adzic et al. [42] examined the
properties of a homologous series of alloys with a composition corresponding
to La 1−x Ce x Ni 3.55 Co .75 Mn .4 Al .3 and measured their comparative performance as
battery electrodes. A PCT diagram for this system is shown in Figure 9.10. Note
that at x > 0.2 there is a decrease in the H storage capacity and thermodynamic
stability until at x = 1 the decreases in both parameters are marked. This reduced
stability is not unexpected as the unit cell volume decreases with Ce content (see
Figure 9.7).
Cycle life plots for the La 1−x Ce x B 5 electrodes are illustrated in Figure 9.11. The
decreased charge capacity found in all La 1−x Ce x B 5 alloys with x > 0.35 conforms to
the shorter and higher plateau pressures of the isotherms depicted in Figure 9.10.
The extremely low electrochemical capacity of CeB 5 is a consequence of the high
dissociation pressure of the hydride phase.
The corrosion rates for the La 1−x Ce x B 5 electrodes are listed in Table 9.5. The
results are summarized graphically in Figure 9.12, which plots lattice expansion,