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5
Nickel Hydroxides

James McBreen

5.1
Introduction

Nickel hydroxides have been used as the active material in the positive electrodes
of several alkaline batteries for over a century [1]. These materials continue to
attract much attention because of the commercial importance of nickel–cadmium
and nickel–metal hydride batteries. In addition to being the active material of
the cathode in nickel–metal hydride batteries, Ni(OH) 2 is an important corrosion
product of the anode during cycling. There are several reviews of work in this field
[2–10].
Progress in understanding the reactions of nickel hydroxide electrodes has been
very slow because of the complex nature of these reactions. Exercises which are
normally trivial for most battery electrodes, such as the determination of the
open-circuit potential, the overall reaction, and the oxidation state of the charged
material, have required much effort and ingenuity. The materials have been studied
with the aid of an enormous array of spectroscopic, structural, and electrochemical
techniques. The most significant advance in the understanding of the overall
reaction was made by Bode and his co-workers [11]. They established that both the
discharged material (Ni(OH) 2 ) and the charged material (NiOOH) could exits in two
forms. One form of Ni(OH) 2 , which was designated as β-Ni(OH) 2 , is anhydrous and
has a layered brucite (Mg(OH) 2 ) structure. The other form, α-Ni(OH) 2 ,ishydrated
and has intercalated water between brucite-like layers. Oxidation of β-Ni(OH) 2 on
charge produces γ -NiOOH, and oxidation of α-Ni(OH) 2 also produces γ -NiOOH.
Discharge of β-NiOOH yields β-Ni(OH) 2 , and discharge of γ -NiOOH yields
α-Ni(OH) 2 . During voltage hold in discharge mode, the α-Ni(OH) 2 can dehydrate
and recrystallize in the concentrated alkaline electrolyte to form β-Ni(OH) 2 . Bode
et al. also found that β-NiOOH could be converted to γ -NiOOH when the electrode
is overcharged. Their overall reaction scheme is shown schematically in Figure 5.1.
All subsequent work has in general validated these conclusions. The two reaction
schemes are often referred to as the β/β and the α/γ cycles.

Handbook of Battery Materials, Second Edition. Edited by Claus Daniel and J¨ urgen O. Besenhard.
 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.