Environmental Engineering Reference
In-Depth Information
vehicles applications. They are characterized by an specific energy of 35-50 Wh/kg
and specific power higher than 150 W/kg, and are called ''no maintenance''
battery, because they require minimal attention or low maintenance by the user.
Two types of VRLA batteries are available commercially: the first is based on
absorbed glass mat (AGM) and the second on the gel technology. AGM batteries
have the best performance, because they have the lowest internal resistance,
whereas their specific gravity is about 1.2 times the flooded Pb-acid and about 1.3
the VRLA gel batteries, but several critical factors such as ventilation, space
requirements, acid containment, safety, environmental impact, material recycla-
bility have to be carefully taken in consideration by the designer.
5.3.2.2 Nickel-Zinc Batteries
Nickel oxide electrodes constitute the positive plates of several storage systems
(among which nickel-zinc, nickel cadmium, nickel metal-hydride, sodium-nickel
chloride) [
16
]. In recent years, the high-specific energy and specific power of Ni-Zn
systems has increased the interest in their use for electric vehicles with respect to
the past years, when their application was essentially limited by a short cycle life.
In these systems, the discharge semi-reaction occurring at the nickel electrode is:
2NiOOH
þ
2H
2
O
þ
2e
!
2Ni O
ð
2
þ
2OH
ð
5
:
9
Þ
while the zinc electrode is involved in the following dissolution-precipitation
semi-reactions:
Zn
þ
4OH
!
Zn O
ð
2
4
þ
2e
ð
i
Þ
ð
5
:
10
Þ
Zn O
ð
2
4
!
ZnO
þ
2OH
þ
H
2
O
ð
ii
Þ
ð
5
:
11
Þ
The electrolyte is a solution of aqueous KOH (usually 20-35 wt%) with 1 wt%
of LiOH (which is saturated with ZnO), whose function is to enhance the charge
acceptance of NiOOH electrode. The overall discharge reaction is the following:
Zn
þ
2NiOOH
þ
H
2
O
!
ZnO
þ
2Ni O
ð
2
ð
5
:
12
Þ
with a cell voltage of 1.75 V at room temperature. During the charge phase ZnO
and Ni(OH)
2
are dissolved with re-deposition onto the electrodes of zinc and
nickel oxide.
This type of batteries is characterized by a specific energy of about 70 Wh/kg
and a specific power of about 150 W/kg, great temperature tolerance, from -39 to
81C, and a quire flat discharge characteristic. Nevertheless, the key problem for
these batteries was the maximum number of deep discharge cycles, principally
caused by the high solubility of the zinc oxide. After years of development, the
Ni-Zn batteries have today reached the commercial viability, and present per-
formance comparable to those of Ni-Cd and Ni-MH systems.
