Panasonic HHR160A User Manual
NICKEL METAL HYDRIDE BATTERIES
Overview
As electronic products have come to feature more
sophisticated functions, more compact sizes and
lighter weights, the sources of power that operate
these products have been required to deliver increasingly higher levels of energy . To meet this
requirement, nickel-metal hydride batteries have
been developed and manufactured with nickel hydroxide for the positive electrode and hydrogen-absorbing alloys, capable of absorbing and releasing hydrogen at high-density levels , for the negative electrode.
Because Ni-MH batteries have about twice the
energy density of Ni-Cd batteries and a similar
operating voltage as that of Ni-Cd batteries, they are
expected to become a mainstay in the next generation of rechargeable batteries.
Construction
Nickel-metal hydride batteries consist of a positive
plate containing nickel hydroxide as its principal active
material, a negative plate mainly composed of hydrogen-absorbing alloys, a separator made of fine fibers,
an alkaline electrolyte, a metal case and a sealing
plate provided with a self-resealing safety vent. Their
basic structure is identical to that of Ni-Cd batteries.
With cylindrical nickel-metal hydride batteries, the
positive and negativ e plates are seperated by the
separator , wound into a coil, inserted into the case ,
and sealed by the sealing plate through an electrically
insulated gasket.
With prismatic nickel-metal hydride batteries, the
positive and negative plates are sandwiched together
in layers with separators between them, inserted into
the case, and sealed by the sealing plate.
NICKEL METAL HYDRIDE HANDBOOK, PA GE
7
August 2000
NICKEL METAL HYDRIDE BATTERIES - CONTINUED
Structure of Nickel Metal Hydride Batteries
Cap (+)
Positive
Electrode
Collector
Case
( )
Insulator
Safety V ent
Cylindrical T ype
Sealing Plate
Insulation Ring
Negative Electrode
Separator
Positive Electrode
Insulation Ring
Insulator
Negative Electrode
Case
Prismatic T ype
Principle of Electrochemical Reaction Involved in Batteries
Hydrogen-absorbing Alloys
Hydrogen-absorbing alloys have a comparatively
short history which dates back about 20 years to the
discovery of NiFe, MgNi and LaNi
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 exothermically with metal (B) whose hydrides generate heat
endothermically to produce the suitable binding
energy so that hydrogen can be absorbed and released at or around normal temperature and pressure
levels. Depending on how metals A and B are combined, the alloys are classified into the following
types: AB (TiFe, etc.), AB
etc.) and A
2
B (Mg2Ni, etc.). From the perspective of
2
(ZnMn2, etc.), AB5 (LaNi5,
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
titanium and nickel serve as the host metals.
Panasonic is now focusing its attention on AB
alloys which feature high capacity, excellent charge
and discharge efficiency, and excellent cycle life. It
has developed, and is now employing its own MmNi
alloy which uses Mm (misch metal = an alloy consisting of a mixture of rare-earth elements) for metal A.
5
alloys. They are
type alloys in which the
5
type
Principle of Electrochemical Reaction
Involved in Batteries
Nickel-metal hydride batteries employ nickel hydroxide for the positive electrode similar to Ni-Cd batteries. 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 .
Positive
electrode
Negative
electrode
Overall
reaction
(
:
M
Ni(OH)
:
:
M
:M
Ni
hydrogen-absorbing alloy;
+
2
++
HO MH
2
()
+
OH
2
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
5
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 understanding the principle, the figure shows how the
reactions proceed by the transfer of protons (H
Charge
-
OH
Discharge
Charge
--
e
Discharge
Charge
Discharge
Cap
Safety V ent
Sealing Electrode
Positive Electrode
Separator
NiOOH
NiOOH
H
ab
+
+
OH
ab
+
MH
:
absorbed hydrogen)
HO
2
ab
-
+
e
+
).
NICKEL METAL HYDRIDE HANDBOOK, PA GE
8
August 2000
Loading...