Page 361 - Handbook of Battery Materials

11.3 Separators for Alkaline Storage Batteries 331

electrode to discharge cadmium, which was already charged. As a prerequisite the
separator has to be permeable to gaseous oxygen; this is achieved by separator
pores being of a specific minimum size and not all of them being filled with
electrolyte at the same time, so as to leave some gas channels. For this application
the fleeces of polyamide, polyethylene, or polypropylene fibers mentioned above
have proven themselves. With their porosity they can absorb sufficient electrolyte,
and due to their pore size distribution they can simultaneously bind electrolyte
and allow oxygen transfer.
Mechanical strength becomes an important criterion, because wound cells
(spiral-type construction), in which a layer of separator material is spirally wound
between each pair of electrodes, are manufactured automatically at very high speed.
Melt-blown polypropylene fleeces, with their excellent tensile properties, offer an
interesting option. Frequently, two layers of the same or different materials are
used, to gain increased protection against shorts; for button cells the use of three
layers, even, is not unusual. Nevertheless the total thickness of the separation does
◦
not exceed 0.2–0.3 mm. For higher-temperature applications (up to about 60 C)
polypropylene fleeces are preferred since they offer better chemical stability, though
at lower electrolyte absorption [113].

11.3.3.2 Nickel–Metal Hydride Batteries
Cadmium presents an environmental risk. Since small nickel–cadmium cells are
often not separately disposed of, they may enter municipal garbage incinerators.
The search for alternative materials for the negative electrode led to metal hydrides,
which not only are regarded as environmentally less critical, but also allow
higher energy density than cadmium. This is especially important for use in
portable equipment, such as cellular phones or laptop computers, where the
nickel–metal hydride system is especially successful. Only in applications requiring
high current densities are they second to nickel–cadmium. The requirements for
the separators are largely identical with those for the sealed nickel–cadmium cells;
therefore mostly the same separator materials are used. They are described in
Section 11.3.5.

11.3.4
Zinc Systems
11.3.4.1 Nickel–Zinc Storage Batteries
Electrochemical systems with zinc as the negative electrode material in alkaline
electrolyte promise high energy and power densities. The nickel–zinc storage
battery especially is being discussed as a candidate for the power source of electric
vehicles, last but not least because zinc – compared with the above-mentioned
metal hydrides – is of low cost and available in sufficient quantity. Even though this
system has been studied and developed since 1930 [114], no success has yet been
achieved in reaching a sufficient number of cycles, so no commercial utilization
has resulted; 200–300 cycles are still considered to be the limit today, although
recently laboratory cells are reported to have reached 600 cycles [115].