Page 295 - Handbook of Battery Materials

9.7 Summary 265

La 0.8 Ce 0.2 Ni 4.8 Sn 0.2 and LaNi 4.8 Sn 0.2 electrodes using in situ XAS. It was determined
by analysis of the X-ray absorption near-edge structure (XANES) that the presence
of Ce reduced Ni corrosion – a finding which confirmed previous cycle life
experiments [42]. This was done by quantitatively determining the amount of
oxidized Ni (assumed to be Ni(OH) 2 ) in cycled electrodes as a function of Ce
content. It is of interest to note the 001 peak of α Ni(OH) 2 was weakly observed in
an electrode after 500 cycles using conventional X-ray diffraction (XRD). While this
is to be expected, since the nickel hydroxide formed is somewhat amorphous, it
illustrates an important advantage of XAS over XRD, since the former probes short
range order and thus can provide quantitative information regarding amorphous
or partly amorphous materials. Tryk et al. have similarly examined LaNi 5 [66] and
MmNi 3.5 Co .8 Mn .3 Al .4 [67] electrodes and noted the electronic transitions taking
place in the metal lattice as a function of charge and the strong interaction of
absorbed H with Ni. This is not unexpected as hydrogen occupies an Ni tetrahedral
site in LaNi 5 H 6 (Figure 9.5).
XAS studies have also been carried out on C14 Laves phase alloys Ti 0.5 Zr 0.5 M 2
and Ti 0.75 Zr 0.25 M 2 (M = V 0.5 Ni 1.1 Fe 0.2 Mn 0.2 ) [61]. The XANES spectra at the Ni K
edge indicates that, unlike the AB 5 alloys, there is very little interaction between
hydrogen and Ni but rather strong interactions with Ti, V, and Zr. The hydrogen
is presumably located in tetrahedra that contain large fractions of these three
elements, whereas the Ni-rich sites are probably empty. Thus the function of Ni
in AB 2 alloys may be primarily to serve as a catalyst for the electrochemical and
hydriding reactions.

9.7
Summary
This survey presents an overview of the chemistry of metal-hydrogen systems which
form hydride phases by the reversible reaction with hydrogen. The discussion then
focuses on the AB 5 class and, to a lesser extent, the AB 2 class of MHs, both of
which are of interest for battery applications. A new section has been introduced on
super-stoichiometric La(Ni, Sn) 5+x electrodes, which have a higher storage capacity
and cycle life than commercial-type electrodes containing Co.
Electrode corrosion is the critical problem associated with the use of MH anodes
in batteries. The extent of corrosion is essentially determined by two factors: alloy
expansion and contraction in the charge–discharge cycle and chemical surface
passivation via the formation of corrosion resistant oxides or hydroxides. Both
factors are sensitive to alloy composition, which can be adjusted to produce
electrodes having an acceptable cycle life. In AB 5 alloys the effects of Ce, Co, Mn,
and Al upon cycle life in commercial type AB 5 electrodes are correlated with lattice
expansion and charge capacity. Ce was shown to inhibit corrosion even though
lattice expansion increases. Co and Al also inhibit corrosion. XAS results indicate
that Ce and Co inhibit corrosion via surface passivation.
There are few systematic guidelines which can be used to predict the properties
of AB 2 MH electrodes. Alloy formulation is primarily an empirical process where