Page 190 - Handbook of Battery Materials

5.3 Solid-State Chemistry of Nickel Hydroxides 159

5.3.2
Pyroaurite-Type Nickel Hydroxides
Allmann found that when suitable trivalent ions were introduced during the
precipitation of the hydroxides of Mg, Zn, Mn, Fe, Co, and Ni, these were
incorporated in the lattice, and the structure changed from the brucite (Mg(OH) 2 )
to the pyroaurite (Mg 6 Fe 2 (OH) 16 (CO 3 )·4H 2 O) type of structure [68]. One of the
nickel materials he prepared was an Ni/Al hydroxide. Axmann et al. [69–71] have
given the nickel compounds the general formula
III
[Ni II M (OH 2 )] [(X ) x/n (H 2 O) z ] x−
x+
n−
x
1−x
K x+ A x−
where 0.2 ≤ x ≤ 0.4 and X n− are the anions of the precursor salts. The resultant
structure consists of brucite cationic layers intercalated with anions and water
molecules. The cationic nature of the brucite layers is due to the higher valence
of the substituent cations, and the anions in the intercalated anion layers pro-
vide electro-neutrality. As a result, the interlayer distance increases from 4.68 to
7.80 ˚ A, when compared with β-Ni(OH) 2 . The structure is shown schematically
in Figure 5.4. Electrochemical and chemical oxidation transforms the pyroaurite
structure to a product that is isostructural with γ -NiOOH [72]. In the early work,
the pyroaurite compounds were prepared by precipitation. Buss et al. report on
the preparation of Ni 4 Al(OH) 10 NO 3 prepared in a computer-controlled apparatus
−1
wherein a solution of 0.4 mol L −1 Ni(NO 3 ) 2 6H 2 O + 1mol L Al(NO 3 ) 3 ·9H 2 Oin
doubly distilled water with the pH adjusted to 2 with 1 mol L −1 HNO 3 was simul-
taneously sprayed with a carbonate-free 1 mol L −1 KOH solution into a receptor
◦
solution maintained at pH 11.5 and a temperature of 32 C [70, 71]. The precipitate
◦
was filtered, washed, and dried at 50 C for three days at 0.01 bar and carbon
dioxide was excluded at all stages of the preparation.
Delmas and his co-workers have done extensive work on pyroaurite-type mate-
rials, which has recently been reviewed [74]. In addition to precipitation methods,
they have prepared the materials by mild oxidative hydrolysis of nickelates that were
prepared by thermal methods similar to those used for the preparation of LiNiO 2
[75]. A cobalt-substituted material NaCo x (Ni 1−x O 2 ) was prepared by the reaction
◦
of Na 2 O, Co 3 O 4 , and NiO at 800 C under a stream of oxygen. The material was
then treated with a 10 mol L −1 NaClO + 4mol L −1 KOH solution for 15 h to form
the oxidized γ -oxyhydroxide. The pyroaurite phase was prepared by subsequent
reduction in a solution of 0.1 mol L −1 H 2 O 2 in 4 mol L −1 KOH [76]. These mild
chemical treatments are referred to as ‘chemie douce’ reactions [69]. The thermally
prepared nickelates have a layered R3m structure. The ‘chemie douce’ treatments
essentially leave the covalently bonded Co x Ni 1−x O 2 layers intact, as a result of
which more crystalline materials with larger particle sizes can be made by this
method.
Most of the work on pyroaurite materials has been done on materials with Fe
[68–73, 77], Co [68, 76, 78], Mn [72, 79], or Al [68, 70, 72] substitutions. When at
least 20% of the Ni atoms are replaced by the trivalent substituent, the materials
are stable in concentrated KOH. In many ways, the pyroaurite phase is similar to