Page 182 - Handbook of Battery Materials

5.3 Solid-State Chemistry of Nickel Hydroxides 151

were achieved with the addition of 10% CoO [13, 15]. The following mechanism
has been proposed for this improvement [13]. In the alkaline electrolyte, CoO
dissolves to form the blue cobaltite ion. The ion precipitates on the Ni(OH) 2
particles to form insoluble β-Co(OH) 2 .Oncharge, the β-Co(OH) 2 is oxidized to
a highly conductive β-CoOOH, which is not reduced on subsequent discharges.
The β-CoOOH provides interparticle contact and access of electrons to the active
material.
Considerable increases in the capacity density of the electrodes have been
achieved through the use of high-density β-Ni(OH) 2 with a uniform particle
−3
size, a narrow range of pore sizes, and a high tapped density (1.9–2.0 g cm )
[15, 16]. Conventional β-Ni(OH) 2 consists of irregular particles with 30% in-
ner pore volume, a large range of pore sizes, and a tapped density of ∼1.6 g
−3
cm . With the new material, it is possible to increase the active material filling
by 20%. Using this material, it was possible to make electrodes with capacity
−3
densities exceeding 550 mAh cm . Conventional sintered plates have capacity
−3
densities of ∼400 mAh cm .
A major problem with pasted-plastic-bonded and fiber mat electrodes is swelling
of the electrode on cycling. This is due to the formation of γ -NiOOH. This
causes comminution of the active material and an increase in the pore volume.
This problem can be largely avoided by the use of β-Ni(OH) 2 containing ∼7% of
coprecipitated Cd or Zn. The coprecipitation of one of these additives along with
7% Co also greatly improves the charge acceptance of the electrode at elevated
◦
temperatures (up to 45 C) [15]. This additive combination also greatly improves
the charge retention at elevated temperatures [15]. Zinc is preferred over Cd as
an additive because of its lower toxicity and the detrimental effect of Cd on metal
hydride electrodes. These advances in the nickel hydroxide electrode represent
considerable progress, and have increased the capacity of sealed nickel–cadmium
AA cells, at the C rate, from 500 to 800 mAh [15].

5.3
Solid-State Chemistry of Nickel Hydroxides

5.3.1
Hydrous Nickel Oxides

β-Ni(OH) 2 , α-Ni(OH) 2 , β-NiOOH, and γ -NiOOH are considered to be the model
divalent and trivalent materials for the nickel hydroxide electrode.

5.3.1.1 β-Ni(OH) 2
β-Ni(OH) 2 can be made with a well-defined crystalline structure and is in many
ways similar to the active material in chemically prepared battery electrodes
that are made by the method described by Fleischer [17]. Several methods of
◦
preparation have been reported. One is to precipitate the hydroxide at 100 Cfrom
a nickel nitrate solution by addition of a KOH solution. Further enhancement