Page 280 - Handbook of Battery Materials

250 9 Metal Hydride Electrodes

have described both laboratory and industrial preparation techniques for many
intermetallic hydride formers, particularly emphasizing LaNi 5 and its substituted
analogs [36].
All AB 5 alloys are very brittle and are pulverized to fine particles in the
hydriding–dehydriding process (see Section 9.3.1, Alloy Activation). Thus, elec-
trodes must be designed to accommodate fine powders as the active material.
There are several methods of electrode fabrication: Sakai et al. [35] pulverize the
alloy by subjecting it to several hydrogen absorption–desorption cycles, then coat
the resulting particles with Ni by chemical plating. The powder is then mixed with
a Teflon dispersion to get a paste, which is finally roller pressed to a sheet and
then hot pressed to an expanded nickel mesh. The fabrication of a simple paste
electrode suitable for laboratory studies is reported by Petrov et al. [37].

9.3.2.3 Effect of Temperature
Ni–MH batteries are currently under consideration for use as power sources
for automotive propulsion and thus will be required to operate over a large
ambient temperature range. The stated goal of the USABC (US Automotive
Battery Consortium) program [38] is to develop a battery which can operate
◦
satisfactorily over a range extending from −30 to 65 C. However, hydride stability
is a logarithmic function of the temperature and must be taken into account when
choosing an electrode composition. For example, the equilibrium plateau pressure
◦
(decomposition) of LaNi 5 H x at 65 C ≈ 10 atm – much too high for use as a
battery electrode. Van’t Hoff plots [10] for LaNi 5 H x ,MmNi 3.55 Co .75 Mn .4 Al .3 H x and
∗
Mm Ni 3.55 Co .75 Mn .4 Al .3 H x (Mm = cerium free mischmetal, see Table 9.2) are
∗
◦
showninFigure9.8.At65 the absorption plateau pressure of Mm B 5 would be
∗
0.5 atm, whereas that of MmB 5 is 5.0 atm. Thus, even though both mischmetal
electrodes have similar electrochemical properties at room temperature, only the
former would be suitable for use at higher temperatures.
9.3.3
Electrode Corrosion and Storage Capacity

Deterioration of electrode performance due to corrosion of electrode components
is a critical problem. The susceptibility of MH x electrodes to corrosion is essentially
determined by two factors, surface passivation due to the presence of surface
oxides or hydroxides and the molar volume of hydrogen, V H , in the hydride
phase. As pointed out by Willems and Buschow [39], V H is important since
it governs alloy expansion and contraction during the charge–discharge cycle.
Large volume changes increase the flushing action of the electrolyte through the
pores and micro-cracks of the electrode during each charge and discharge cycle,
thereby increasing the rate of contact of the alloy surface with fresh electrolyte
and, consequently, the corrosion rate. Thus, when examining the effect of various
substituents upon electrode corrosion the question always arises whether an
observed change is due to a change in lattice expansion or to a change in surface
passivation, for example, the formation of a corrosion-resistant oxide layer.