Page 195 - Handbook of Battery Materials

164 5 Nickel Hydroxides

5.4.3
Nickel Oxidation State
Like all other facets of the electrode, determination of the overall redox process has
been difficult and many aspects are still disputed. The presence of Ni(IV) species
in charged materials has been proposed by many authors. The early work has been
reviewed [9]. The evidence for Ni(IV) is based mostly on coulometric data [95] or
determinations of active oxygen by titration with iodide or arsenious oxide. Active
oxygen contents corresponding to a nickel valence of 3.67 have been reported for
α-Ni(OH) 2 films charged in 1 mol L −1 KOH [95], and values of 3.48 were found
for overcharged β-Ni(OH) 2 battery electrodes in 11 mol L −1 KOH [96]. When these
electrodes of high active oxygen content are discharged, or even over-discharged, an
appreciable amount of active oxygen remains [57]. Cycling between a nickel valence
of 2.5 and 3.5 has been proposed [97]. X-ray absorption has been used to study the
problem [98, 99]. In one case, results consistent with an Ni oxidation state of 3.5
were found for a charged electrode [99]. In the case of the α/γ couple, indications
are that a nickel oxidation state of at least 3.5 can be reached on charge. It is not clear
that this is the case with the β/β couple. In situ experiments with simultaneous
X-ray diffraction and X-ray absorption measurements should be done on the β/β
couple to check for the presence of γ -NiOOH. Experiments on materials stabilized
with both Co and Zn additives are also necessary.
The existence of these high nickel oxidation states offers the possibility of a
‘two-electron’ electrode. This is one of the incentives for stabilizing the α/γ cycle
through the use of the pyroaurite structures [74]. With this approach, it has been
possible to achieve a 1.2 electron exchange for the overall reaction. However,
none of the pyroaurite structures is satisfactory for battery electrodes. The Co-
and Mn-substituted materials are unstable with cycling [72, 74]. The end-of-charge
voltages for both the Fe- and Al-substituted materials are high and the charging
efficiencies are low [72, 74]. However, the use of mixed substitution, such as
combinations of Co and Al, can lower the charging voltage [74].

5.4.4
Oxygen Evolution

Oxygen evolution occurs on nickel oxide electrodes throughout charge, on over-
charge, and on standby. It is the anodic process in the self-discharge reaction of
the positive electrode in nickel–cadmium cells. Early work in the field has been
reviewed [9]. No significant new work has been reported in recent years.

5.4.5
Hydrogen Oxidation

The reaction of hydrogen at the nickel electrode determines the rate of self-discharge
in nickel–hydrogen batteries.