Page 189 - Handbook of Battery Materials

158 5 Nickel Hydroxides

5.3.1.4 γ -NiOOH
γ -NiOOH is the oxidation product of α-Ni(OH) 2 . It is also produced on overcharge
of β-Ni(OH) 2 , particularly when the charge is carried out at high rates in high
concentrations of alkali [11, 59]. Use of the lighter alkalis (LiOH and NaOH)
favor the formation of γ -NiOOH, whereas use of RbOH inhibits its formation
[60]. The material was first prepared by Glemser and Einerhand [52] by fusing
◦
one part Na 2 O 2 with three parts NaOH in a nickel crucible at 600 C. Hydrolysis
of the product yields γ -NiOOH. They gave cell dimensions for a rhombohedral
system with a = 2.8 ˚ A and c = 20.65 ˚ A. The material has a layer structure with
a spacing of 7.2 ˚ A between layers. γ -NiOOH always contains small quantities of
alkali metal ions and water between the layers, whereas β-NiOOH does not. The
X-ray diffraction patterns have more, and much sharper, lines than those of either
α-Ni(OH) 2 or β-NiOOH [19, 53]. γ -NiOOH prepared by the method of Glemser
and Einerhand has the formula NiOOH·0.51H 2 O. TGA analysis shows that this
◦
water is lost between 50 and 180 C [58].

5.3.1.5 Relevance of Model Compounds to Electrode Materials
The reaction scheme of Bode [11] was derived by comparison of the X-ray diffraction
patterns of the active materials with those for the model compounds. How the
β-Ni(OH) 2 in battery electrodes differs from the model compound is discussed
in Section 5.3.1.3. In recent years, the arsenal of in situ techniques for electrode
characterization has greatly increased. Most of the results confirm Bode’s reaction
scheme and essentially all the features of the proposed α/γ cycle. For instance,
recent atomic force microscopy (AFM) of α-Ni(OH) 2 shows results consistent with
a contraction of the interlayer distance from 8.05 to 7.2 ˚ A on charge [61–63]. These
are the respective interlayer dimensions for the model α-Ni(OH) 2 and γ -NiOOH
compounds. Electrochemical quartz crystal microbalance (ECQM) measurements
also confirm the ingress of alkali metal cations into the lattice upon the conversion of
α-Ni(OH) 2 to γ -NiOOH [45, 64, 65]. However, in situ Raman and surface-enhanced
Raman spectroscopy (SERS) results on electro-chemically prepared α-Ni(OH) 2 in
1 mol L −1 NaOH show changes in the O–H stretching modes that are consistent
with a weakening of the O–H bond when compared with results for the model
α-and β-Ni(OH) 2 compounds [66]. This has been ascribed to the delocalization
+
of protons by intercalated water and Na ions. Similar effects have been seen in
passive films on nickel in borate buffer electrolytes [67].
Recent ECQM work and X-ray diffraction have confirmed the conversion of the
α/γ cycle to the β/β cycle upon electrochemical cycling in concentrated alkali.
Earlier ECQM studies of α-Ni(OH) 2 films had shown a mass inversion in the
microgravimetric curve after prolonged cycling [64]: there is a mass decrease in
charge instead of a mass increase. More recent work has confirmed that this mass
inversion is due to conversion of the α/γ cycle to the β/β cycle [65].