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Ni-MH Rechargeable Batteries
3
3
Composition and Chemistry
A rechargeable battery is based on the principle that the charge/discharge process is reversible, that is, the
energy delivered by the battery during discharge can be replaced or restored by recharging.
Nickel oxyhydroxide (NiOOH) is the active mate-
rial in the positive electrode of the nickel-metal hydride
battery in the charged state, the same as in the nickel-
cadmium battery.
The negative active material, in the charged state,
is hydrogen in the form of a metal hydride. This metal
alloy is capable of undergoing a reversible hydrogen
absorbing/desorbing reaction as the battery is charged
and discharged, respectively.
The unique attribute of the hydrogen storage
alloy is its ability to store hundreds of times its own
volume of hydrogen gas at a pressure less than atmos-
pheric pressure. Many different intermetallic com-
pounds have been evaluated as electrode materials for
nickel-metal hydride batteries. Typically, these fall into
two classes: AB
5
alloys, of which LaNi
5
is an example,
and AB
2
alloys, of which TiMn
2
or ZrMn
2
are examples.
DURACELL nickel-metal hydride battery technol-
ogy is based on the use of AB
5
instead of AB
2
alloys.
AB
5
alloys offer better corrosion resistance characteris-
tics, resulting in longer cycle life and better recharge-
ability following storage. The composition of the metal
alloy is formulated for optimal stability over a large
number of charge/discharge cycles. Other important
properties of the alloy include:
•
Large hydrogen storage capability for high energy
density and battery capacity.
•
Favorable kinetic properties for high rate capability
during charge and discharge.
•
Low hydrogen pressure alloy and high purity mate-
rials to minimize self-discharge.
An aqueous solution of potassium hydroxide is
the major component of the electrolyte of a nickel-
metal hydride battery. A minimum amount of elec-
trolyte is used in this sealed cell design, with most of
this liquid being absorbed by the separator and the
electrodes. This “starved electrolyte” design facilitates
the diffusion of oxygen to the negative electrode at the
end-of-charge for the “oxygen recombination” reaction.
During discharge, the nickel oxyhydroxide is
reduced to nickel hydroxide
NiOOH + H
2
O + e
-
——> Ni(OH)
2
+ OH
-
and the metal hydride (MH) is oxidized to the metal
alloy (M).
MH + OH
-
——> M + H
2
O + e
-
The overall reaction on discharge is:
MH + NiOOH ——> M + Ni(OH)
2
The process is reversed during charge.
3.3 Cell Reactions
3.2 Electrolyte
3.1 Active Components: Positive and Negative Electrodes
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