Page 276 - Handbook of Battery Materials

246 9 Metal Hydride Electrodes

of La associated with oxygen and Ni clusters. This is the mechanism by which
catalytic metal surface sites are formed for chemical or electrochemical reactions.
The activation procedure is straightforward; in gas/solid systems it merely consists
of repeated formation and decomposition of the hydride phase [20]. Electrochemical
activation also consists of repeated charge and discharge cycles which sometimes
require extended cycling periods. Alloys may be activated via the gas–solid reaction
and then used as an electrode; this can significantly shorten the period required
for electrochemical activation.

9.3.2
AB 5 Electrodes

The use of LaNi 5 as an electrode was first reported by Justi in 1973 [21]. However,
−1
the capacity was less than 1/3 that of 372 mAh g , which corresponds to the
discharge of six hydrogens from LaNi 5 H 6 . This was primarily due to the fact that
the dissociation pressure of LaNi 5 H 6 exceeds 1 atm at 298 K. Thus, in an open
cell most of the hydrogen is lost as H 2 gas. A few years later Percheron-Guegan
and co-workers [22] substituted Al and Mn in part for nickel, thereby increasing
hydride stability and charge capacity. However, this did not significantly affect
the rapid corrosion of the electrode as observed with LaNi 5 . In 1984 Willems [23]
prepared the first multi-component AB 5 electrode that had an acceptable cycle life.
He also reported the positive correlation between lattice expansion and electrode
corrosion. Finally in 1987 an alloy of composition MmNi 3.55 Co 0.75 Mn 0.4 Al 0.3 was
shown to meet the minimum requirements for a practical battery with respect to
cost, cycle life, and storage capacity [24]. Indeed, this composition is very similar
to those currently used in commercial Ni–MH batteries with AB 5 hydride anodes.
Ikoma et al. [25] describe an experimental electric vehicle (EV) battery having an
energy density of 70 Wh kg −1 using an anode of composition Mm(Ni,Co,Mn,Al) 5 .
The electrochemical behavior of this alloy and its relation to small changes in alloy
composition is of great practical interest and will be discussed at length in the
following sections.

9.3.2.1 Chemical Properties of AB 5 Hydrides
In order to fully understand the electrochemical behavior of AB 5 hydrides a
knowledge of their chemical properties is required. Van Vucht et al. [7] were the
first to prepare LaNi 5 hydride, and this is arguably the most thoroughly investigated
H storage compound. It reacts rapidly with hydrogen at room temperature at a
pressure of several atmospheres above the equilibrium plateau pressure. PC
isotherms for this system are shown in Figure 9.3. The nominal reaction may be
written as

(9.10)
LaNi 5 + 3H 2 ⇔ LaNi 5 H 6
LaNi 5 has the CaCu 5 structure; space group P6/mmm [26]; the hexagonal metal
lattice is shown in Figure 9.4. The crystal structure of LaNi 5 D 7 has been determined
[27, 28] and is illustrated in Figure 9.5. There are three types of interstitial D sites,