Page 181 - Handbook of Battery Materials

150 5 Nickel Hydroxides

b-NiOOH Overcharge g-NiOOH Figure 5.1 Reaction scheme of Bode
[11].

Charge Discharge Discharge Charge

Dehydration
b-Ni(OH) 2 a-Ni(OH) 2

This section gives a brief overview of the structure of nickel hydroxide battery elec-
trodes and a more detailed review of the solid-state chemistry and electrochemistry
of the electrode materials. Emphasis is on work done since 1989.

5.2
Nickel Hydroxide Battery Electrodes

Conventional nickel hydroxide battery electrodes are designed to operate on the
β/β cycle, to accommodate the volume changes that occur during cycling, and to
have adequate electronic conductivity to yield high utilization of the active material
on discharge. The β/β cycle is preferred because there is less swelling of the
active material on cycling. The conductivity of β-NiOOH is more than 5 orders
of magnitude higher than that of Ni(OH) 2 [12]. As a result, there is usually no
problem in charging the electrode because the NiOOH that forms increases the
conductivity of the active material. However, on discharge the charged material
can become isolated in a resistive matrix of the discharged product and cannot be
discharged at useful rates [13]. Operation on the β/β cycle is ensured by control of
the electrolyte composition and the use of a combination of additives such as Co
and Zn. Provisions have to be made for electronic conduction to the active material
and confinement of the active material on cycling. Over the years, several electrode
designs have been used. These include incorporation of the active material in pocket
plates, perforated metal tubes, sintered nickel plaques, plastic-bonded electrodes
with graphite as the conductive diluent, nickel foams, and fibrous nickel mats.
Pocket and tubular electrodes have been described in detail by Falk and Salkind
[1]. McBreen has reviewed work on both sintered-plate and plastic-bonded electrode
technology [9]. More recent work is on the use of nickel foams and nickel mats.
Early work on the use of foams and mats has been reviewed [9]. Nickel fiber,
nickel-plated steel fiber, or nickel-plated graphite fiber mats are preferred because
they have smaller pores (∼50 µm) [14]. The most recently developed mats can
have porosities as high as 95% [13] and are much lighter than the sintered nickel
plaques, which typically have porosities between 80 and 90%. Initially, standard
cathodic impregnation methods were used to load the active material into the
foam [9]. More recently, the preferred method is to incorporate the Ni(OH) 2 in
the form of a slurry into the mat [13, 14]. This has been called the ‘suspension
impregnation method’ [14]. Considerable improvement in the Ni(OH) 2 has been
achieved by the addition of divalent Co compounds to the slurry. The best results