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August 2000
NICKEL METAL HYDRIDE HANDBOOK, PAGE
8
Structure of Nickel Metal Hydride Batteries
Principle of Electrochemical Reaction Involved in Batteries
Hydrogen-absorbing Alloys
NICKEL METAL HYDRIDE BATTERIES - CONTINUED
Hydrogen-absorbing alloys have a comparatively
short history which dates back about 20 years to the
discovery of NiFe, MgNi and LaNi
5
alloys. They are
capable of absorbing hydrogen equivalent to about a
thousand times of their own volume, generating metal
hydrides and also of releasing the hydrogen that they
absorbed. These hydrogen-absorbing alloys combine
metal (A) whose hydrides generate heat exothermi-
cally with metal (B) whose hydrides generate heat
endothermically to produce the suitable binding
energy so that hydrogen can be absorbed and re-
leased at or around normal temperature and pressure
levels. Depending on how metals A and B are com-
bined, the alloys are classified into the following
types: AB (TiFe, etc.), AB
2
(ZnMn
2
, etc.), AB
5
(LaNi
5
,
etc.) and A
2
B (Mg
2
Ni, etc.). From the perspective of
charge and discharge efficiency and durability, the
field of candidate metals suited for use as electrodes
in storage batteries is now being narrowed down to
AB
5
type alloys in which rare-earth metals, especially
metals in the lanthanum group, and nickel serve as
the host metals; and to AB
2
type alloys in which the
titanium and nickel serve as the host metals.
Panasonic is now focusing its attention on AB
5
type
alloys which feature high capacity, excellent charge
and discharge efficiency, and excellent cycle life. It
has developed, and is now employing its own MmNi
5
alloy which uses Mm (misch metal = an alloy consist-
ing of a mixture of rare-earth elements) for metal A.
Principle of Electrochemical Reaction
Involved in Batteries
Positive
electrode
:
Negative
electrode
Overall
reaction
Charge
Discharge
Ni(OH)
2
M
hydrogen-absorbing alloy;
absorbed hydrogen)
:
:
:
:M
M
Charge
Discharge
Charge
Discharge
+
+
+
+
+
++
-
--
e
OH
NiOOH
HO MH
ab
HO
NiOOH
ab
ab
2
2
2
OH
OH
Ni
()
MH
H
+
-
e
(
Cylindrical Type
Cap (
+
)
Safety Vent
Sealing Plate
Insulation Ring
Negative Electrode
Separator
Positive Electrode
Case
Positive
Electrode
Collector
( )
Insulator
Insulation Ring
Insulator
Negative Electrode
Case
Separator
Positive Electrode
Sealing Electrode
Safety Vent
Cap
Prismatic Type
Nickel-metal hydride batteries employ nickel hydrox-
ide for the positive electrode similar to Ni-Cd batter-
ies. The hydrogen is stored in a hydrogen-absorbing
alloy for the negative electrode, and an aqueous
solution consisting mainly of potassium hydroxide for
the electrolyte. Their charge and discharge reactions
are shown below.
As can be seen by the overall reaction given above,
the chief characteristics of the principle behind a
nickel-metal hydride battery is that hydrogen moves
from the positive to negative electrode during charge
and reverse during discharge, with the electrolyte
taking no part in the reaction; which means that there
is no accompanying increase or decrease in the
electrolyte. A model of this battery’s charge and
discharge mechanism is shown in the figure on the
following page. These are the useful reactions taking
place at the respective boundary faces of the positive
and negative electrodes, and to assist one in under-
standing the principle, the figure shows how the
reactions proceed by the transfer of protons (H
+
).