Page 184 - Handbook of Battery Materials

5.3 Solid-State Chemistry of Nickel Hydroxides 153

Table 5.1 Crystallographic parameters for β-Ni(OD) 2 [22].

Parameter ( ˚ A)

a 0 3.126
c 0 4.593
Ni–O bond length 2.073
O–H bond length 0.973
Ni–Ni bond length 3.126

are 0, 0, 0 for nickel and 1/3, 2/3, z and 2/3, 1/3, z for oxygen. Values for the
crystallographic parameters of well-crystallized β-Ni(OD) 2 are given in Table 5.1.
Because of anomalous scattering by H, the results for the as-precipitated Ni(OH) 2
could not be refined. Nevertheless, cell constants and the O–H bond distance could
be determined. The results showed that the as-precipitated material was different
from the well-crystallized material. The unit cell dimensions were a 0 = 3.119 ˚ A
and c 0 = 4.686 ˚ A. Also the O–H bond length was 1.08 ˚ A, a value similar to that
previously reported by Szytula et al. in a neutron diffraction study of Ni(OH) 2 [23].
The O–H bond in both well-crystallized and as-precipitated materials is parallel to
the c-axis. The difference between well-crystallized and as-precipitated material is
important since the well-crystallized material is not electrochemically active. The
differences between the materials are attributed to a defective structure that arises
−
from the large concentration of surface OH ion groups in the high-surface-area
material [22]. These are associated with absorbed water. This is a consistent
−1
with an absorption band in the infrared at 1630 cm . This is not seen in the
well-crystallized material.
Infrared spectroscopy has also confirmed the octahedral coordination of nickel
by hydroxyl groups [24, 25]. No evidence for hydrogen bonding has been found. In
battery materials, evidence was also found for a small amount of absorbed water
[24, 26]. Even though these materials contain small amounts of water they are
still classified as β-Ni(OH) 2 because of an (0 0 1) X-ray reflection corresponding
to a d spacing of 4.65 ˚ A. Thermogravimetric analysis (TGA) indicates that the
water is removed at higher temperatures [26–28]. Kober [24, 26] has proposed that
this water is associated with nickel ions in the lattice and suggested the formula
[Ni(H 2 O) 0.326 ](OH) 2 for the chemically prepared battery material. A similar formula
was proposed by Dennsted and Loser [27]. However, this has been disputed [29].
The evidence is that well-crystallized β-Ni(OH) 2 does not contain absorbed water
[22, 29]. However, the high-surface-area material that is used in batteries does. This
is consistent with the expansion in the c-axis of the crystal from 4.593 to 4.686 ˚ A,
the increase in the average O–H bond distance from 0.973 to 1.08 ˚ A [22], and the
presence of broad absorption bond in the infrared spectrum at 1630 cm −l [22, 24,
26]. TGA results indicate that this water is removed in a single process over a
◦
temperature range of 50–150 C [30].