Page 188 - Handbook of Battery Materials

5.3 Solid-State Chemistry of Nickel Hydroxides 157

in an electrochemically impregnated battery electrode is completely converted to
β-Ni(OH) 2 30 min after immersion in 4.5 mol L −1 KOH [36].
Infrared studies of the reaction product in water indicate that the β-Ni(OH) 2 that
is initially formed also contains anions and adsorbed water. As the particle size of
the product increases, the amount of anions and adsorbed water decreases [45].
Delmas and co-workers have proposed the existence of an intermediate phase
between α-and β-Ni(OH) 2 [48]. This phase consist of interleaved α and β material
and can be formed on aging of α-Ni(OH) 2 . Recent Raman results confirm the
existence of such a phase [49].

5.3.1.3 β-NiOOH
β-Ni(OH) 2 has been identified as the primary oxidation product of electrodes
containing β-Ni(OH) 2 [11, 50, 51]. Glemser describes a method for preparation
of β-NiOOH [52, 53]. A solution of 100 g of Ni(NO 3 ) 2 ·6H 2 Oin1.5 LH 2 Owas
added dropwise to a solution of 55 g KOH and 12 mL Br 2 in 300 mL H 2 O, while
◦
keeping the temperature at 25 C. The black precipitate was washed with CO 2 -free
−
water until both K and NO were removed and then dried over H 2 SO 4 . Structural
+
3
determinations of the higher oxides of nickel are complicated because of their
amorphous nature [53]. However, it appears that β-Ni(OH) 2 is oxidized to the
trivalent state without major modifications to the brucite structure. The unit cell
dimensions change from a 0 = 3.126 ˚ A and c 0 = 4.605 ˚ A for β-Ni(OH) 2 to a 0 =
2.82 ˚ Aand c 0 = 4.85 ˚ Afor β-NiOOH. X-ray diffraction clearly indicates expansion
along the c-axis. Asymmetry in the hk lines indicates the turbostratic nature of
β-NiOOH. Even after correcting the a 0 , values for this, McEwen found that there
was a real contraction in the basal plane [19]. Transmission EXAFS has been used
to investigate the oxidation products of β-Ni(OH) 2 [54, 55]. In situ measurements
on plastic-bonded electrodes showed that in the charged state a two-shell fit was
necessary for the first Ni–O coordination shell. This suggests that the oxygen
coordination in β-NiOOH is a distorted octahedral coordination with four oxygens
at a distance of 1.88 ˚ A and two oxygens at a distance of 2.07 ˚ A. The distorted
coordination is consistent with the edge features of the X-ray absorption spectra
[55, 56]. The overall reaction for the electrochemical formation of β-NiOOH is
usually given as

+
β-Ni(OH) 2 + β-NiOOH + H + e − (5.1)
During the reaction, protons are extracted from the brucite lattice. Infrared spectra
[24, 25, 31] show that during charge the sharp hydroxyl band at 3644 cm −1
−1
disappears. This absorption is replaced by a diffuse band at 3450 cm . The spectra
indicate a hydrogen-bonded structure for β-NiOOH with no free hydroxyl groups.
β-NiOOH probably has some adsorbed and absorbed water. However, TGA data
on charged materials are very limited [57, 58], and it is not always clear that the
material is pure β-NiOOH. Unless electrochemical experiments are done very
carefully there is always the possibility of the presence of γ -NiOOH [13].