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Nickel Metal Hydride Battery
The nickel-metal hydride cell chemistry is a hybrid of the
proven positive electrode chemistry of the sealed nickel-cadmium
cell with the energy storage features of metal alloys developed
for advanced hydrogen energy storage concepts. This heritage
in a positive-limited cell design results in batteries providing
enhanced capacities while retaining the well-characterized
electrical and physical design features of the sealed nickel-cadmium
cell design.
Nickel-metal hydride cells, with the exception of the negative
electrode, use the same general types of components as the
sealed nickel-cadmium cell. The basic concept of the nickel-metal
hydride cell negative electrode emanated from research on
the storage of hydrogen for use as an alternative energy
source in the 1970s. Certain metallic alloys were observed
to form hydrides that could capture (and release) hydrogen
in volumes up to nearly a thousand times their own volume.
By careful selection of the alloy constituents and proportions,
the thermodynamics could be balanced to permit the absorption
and release process to proceed at room temperatures and
pressures.
Two general classes of metallic alloys have been identified
as possessing characteristics desirable for battery cell
use. These are rare earth/nickel alloys generally based
around LaNi5 (the so-called AB5 class
of alloys) and alloys consisting primarily of titanium and
zirconium (designated as AB2 alloys). In both
cases, some fraction of the base metals is often replaced
with other metallic elements. The AB5 formulation
appears to offer the best set of features for commercial
nickel-metal hydride cell applications.
The metal hydride electrode has a theoretical capacity
approximately 40 percent higher than the cadmium electrode
in a nickel-cadmium couple. As a result, nickel-metal hydride
cells provide energy densities that are 20-40 percent higher
than the equivalent nickel-cadmium cell.
The nickel-metal hydride positive electrode design draws
heavily on experience with nickel-cadmium electrodes. Electrodes
that are economical and rugged exhibiting excellent high-rate
performance, long cycle life, and good capacity include
pasted and sintered-type positive electrodes.
The balance between the positive and negative electrodes
is adjusted so that the cell is always positive-limited.
This means that the negative electrode possesses a greater
capacity than the positive. The positive will reach full
capacity first as the cell is charged. It then will generate
oxygen gas that diffuses to the negative electrode where
it is recombined. This oxygen cycle is a highly efficient
way of handling moderate overcharge currents.
The electrolyte used in the nickel-metal hydride cell is
alkaline, a dilute solution of potassium hydroxide containing
other minor constituents to enhance cell performance.
The baseline material for the separator, which provides
electrical isolation between the electrodes while still
allowing efficient ionic diffusion between them, is a nylon
blend similar to that currently used in many nickel-cadmium
cells.
The nickel-metal hydride couple lends itself to the wound
construction which is similar to that used by present-day
cylindrical nickel-cadmium cells. The basic components consist
of the positive and negative electrodes insulated by separators.
The sandwiched electrodes are wound together and inserted
into a metallic can that is sealed after injection of a
small amount of electrolyte.
In variation of this design, nickel-metal hydride cells
are also being produced in prismatic versions. The prismatic
cells may fit more easily into volume-critical applications.
The general internal construction of the prismatic cell
is similar to the cylindrical cell except the single positive
and negative electrodes are now replaced by multiple electrode
sets. Thus the trade-off for improved packaging in select
applications is increased complexity in cell assembly with
the corresponding increases in production cost.
Both cylindrical and prismatic nickel-metal hydride cells
are typically two-piece sealed designs with metallic cases
and tops that are electrically insulated from each other.
The case serves as the negative terminal for the cell while
the top is the positive terminal. Some finished cell designs
may use a plastic insulating wrapper shrunk over the case
to provide electrical isolation between cells in typical
battery applications.
Nickel-metal hydride cells contain a resealable safety
vent built into the top. The nickel-metal hydride cell is
designed so the oxygen recombination cycle described earlier
is capable of recombining gases formed during overcharge
under normal operating conditions, thus maintaining pressure
equilibrium within the cell. However, in cases of charger
failure or improper cell/charger design for the operating
environment, it is possible that oxygen, or even hydrogen,
will be generated faster than it can be recombined. In such
cases the safety vent will open to reduce the pressure and
prevent cell rupture. The vent reseals once the pressure
is relieved.
Work Cited
“Nickel-Metal Hydride Application Manual”. Retrieved
19 June 2002 [online]
http://data.energizer.com/batteryinfo/
application/manuals/nickel_metal_hydride.htm
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