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11.3 Separators for Alkaline Storage Batteries  329

               scale; this leads empirically to higher electrical resistance and especially to chemical
               susceptibility. The optimizations achieved to date toward increasing the service
               life of alkaline storage batteries are still unsatisfactory; this presents a particular
               challenge to the further development of separators.

               11.3.2
               Primary Cells

               Primary cells generally do not place high demands on the separator, so these are
               not covered exhaustively here; the lack of a charging process avoids undesirable
               electrochemical deposits (e.g., dendrites) as well as generation of oxidizing sub-
               stances. Thus low-priced, alkali-resistant sheets are used as separators; generally
               cellulosic papers, fleeces, or woven fabrics of poly-amide, poly(vinyl alcohol) (PVA)
               or polypropylene fibers meet this requirement satisfactorily [4]. It is generally suf-
               ficient for them to absorb and retain as much as possible of the electrolyte without
               decomposition and to be resistant against the substance of the positive electrode
               under the conditions of use to be expected. The fleeces of organic fibers are also
               used in alkaline secondary cells and will be explained in more detail in that context
               (cf. Section 11.3.5).

               11.3.3
               Nickel Systems
               11.3.3.1 Nickel–Cadmium Batteries

               11.3.3.1.1 Vented Construction  The first practical alkaline storage batteries
               were developed in the 1890–1910 period by Waldemar Jungner in Sweden and
               almost simultaneously by Thomas Alva Edison in the USA [10]. These nickel–iron
               batteries, because of their high self-discharge rates due to iron poisoning of the
               nickel electrodes, have been replaced almost completely by the nickel–cadmium
               batteries also developed by W. Jungner. The original construction with so-called
               pocket plates is still available today, with only little change. The active material
               powders are held in pockets of perforated (nickel-coated) steel sheets. In the
               simplest case the pocket electrodes are kept at a spacing of about 1–3 mm by PVC
               rod plates (‘ladders’), and occasionally also by extruded PVC ribs or perforated,
               corrugated PVC spacers, according to the designed electrical power performance.
               Since, as mentioned, no soluble ions cause any interferences in a nickel–cadmium
               pocket plate battery, a separator in the narrow sense is not required.
                For increased power requirements, electrode constructions have been developed
               which bring the electronic conductors into closer contact with the active material
               particles: first, around 1930, the sinter electrode [109], recently in sealed cells largely
               replaced by the nickel–foam electrode, and then, around 1980, the fiber structure
               electrode [110]. In order to take full advantage of their increased performance,
               the electrodes have to be as close together as possible; that is, a uniformly thin,
               highly porous separator is required with sufficiently small pores to prevent any
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