Page 255 - Battery Reference Book

Sodium-nickel chloride secondary batteries 1
Cyclic life is anode limited by dendritic growth, zinc This electrode has approximately 40% more theoretical
corrosion, shape change and also zinc poisoning at the capacity than the cadmium negative electrode in a
nickel cathode. nickel-cadmium couple. The metal hydride negative
Yusaza, Japan, have been developing prototype bat- works well with essentially the same nickel positive
teries for electric vehicle applications, claiming 200 electrode used in conventional nickel-cadmium cells,
maintenance-free cycles at 100% depth of discharge. but provides power densities that are 20-30% higher
The more successful, but still unsatisfactory, batter- in the finished cell.
ies such as these are based on the use of PTFE The intrinsic voltage of the couple is approximately
bonded pressed plate zinc electrodes and sintered, plas- the same as with nickel-cadmium: values of 1.2 to
tic bonded pressed plate or fibre or expanded foam 1.35V per cell having been quoted. This helps in
nickel electrodes to improve conductance. maintaining applications compatibility between exist-
Fibre and expanded nickel electrodes are the latest ing nickel-cadmium cells and the new nickel-metal
development and one being studied in Germany and hydride cells.
Japan. The nickel-metai hydride couple lends itself to a
The use of fibre and expanded foam substitutes may
increase the likelihood of a successful commercial wound construction similar to that used by present-day
wound nickel-cadmium cells. The basic componeets
nickel-zinc battery but such batteries will always have consist of the positive and negative electrodes insu-
strong competition from newer types of batteries.
lated by plastic separators similar to those used in
nickel-cadmium products. The sandwiched electrodes
ydride secondary are wound together and inserted into a metallic can
batteries which is sealed after injection of a small amount of
potassium hydroxide electrolyte solution. The result
The basis of the metal hydride technology is the abil- is a cell which bears a striking resemblance to mr-
ity of certain metallic alloys to absorb the smaller rent sealed nickel-cadmium cells. The nickel-metal
hydrogen atoms in the interstices between the larger hydride chemistry is also applicable to prismatic cell
metal atoms. Two general classes of materials have
designs which evoke greater interest as product profiles
been identified a5 possessing the potential for absorb- become thinner.
ing large volumes of hydrogen: rare earthhickel alloys
generally based around LaN& and alloys consisting Quoted practical energy densities for nickel-metal
primarily of titanium and zirconium. In both cases, hydride batteries are markedly superior (25% to 30%)
some fraction of the base metals is often replaced with to those of corresponding nickel-cadmium batteries.
other metallic elements. The exact alloy composition
can be tailored somewhat to accomiiodate the spe-
cific duty requirements of the final cell. Although cells
using both types of materials are appearing on the mar- 19.5 Nickel-iron secondary s
ket. precise alloy formulations are highly proprietary Until recently these batteries have been produced in
and specific to the manufacturer. These metal alloys Germany, the US and Russia in relatively small num-
are used to provide the active materials for the nega- bers, due to poor charge retention and poor efficency
tive electrode in cells which otherwise are very similar in earlier types. Now, due to improvements ir, design.
to a nickel-cadmium cell. they are being produced in larger quantities and are
Use of a metal hydride provides the following reac-
tions at the negative battery electrode: being actively considered for electric vehicle applica-
tions.
The cells have a voltage of 1.4V at 25°C.
Charge
The basic cell reaction is:
When an electrical potential is applied to the cell, in
the presence of the alloy, the water in the electrolyte is Discharge
decomposed into hydrogen atoms which are absorbed Fe(s) + 2NiO(OH)(s) charge Fe(QH)z(s) + 2Ni(OR)p(s)
into the alloy and hydroxyl ion as indicated below. (19.24)
Alloy + I320 + e- ----f Alloy(H) + OH- ( 19.22) Energy densities currently being achieved are
20-30 W h/kg-' (tubular plate electrodes) and
Discharge 40-60 W h/kg-' (sintered plate elecirodes). The pos-
itive plate comprises thick sintered nickel plates on a
During discharge. the reactions are reversed. The nickel plated substrate. The negative plate comprises a
hydrogen is desorbed and combines with an hydroxyl mixture of powdered iron and Fe304. The electrolyte
ion to form water while also contributing an electron contains 1.2 to 1.3 g/~m-~ potassium hydrioxide con-
to the current.
taining 1-2% lithium hydroxide. The cells are vented.
Alloy(W) + OH- + Alloy + H20 + e- (19.23) Synthetic fibres are used for separators.