Page 358 - Handbook of Battery Materials

328 11 Separators

11.3
Separators for Alkaline Storage Batteries
11.3.1
General

In acidic electrolytes, only lead, because it forms passive layers on the active
surfaces, has proven sufficiently chemically stable to produce durable storage
batteries. In contrast, in alkaline media there are several substances basically
suitable as electrode materials: nickel hydroxide, silver oxide, and manganese
dioxide as positive active materials may be combined with zinc, cadmium, iron,
or metal hydrides. In each case potassium hydroxide is the electrolyte, at a
concentration – depending on battery systems and application – in the range of
−3
1.15–1.45 g cm . Several electrochemical couples consequently result, which are
available in a variety of constructions and sizes, with an even larger variety of
separators, of course.
For alkaline storage batteries, requirements often exceed by far those for lead
storage batteries. The reason is that the suitable materials for the positive electrode
are very expensive (silver oxide, nickel hydroxide), and thus the use of these
storage batteries is only justified where requirements as to weight, number of
cycles, or temperature range prohibit other solutions. Besides a few standardized
versions – mainly for nickel–cadmium batteries – this has led to the existence of a
large diversity of constructions for special applications [4–6, 107, 108].
In order to classify this diversity from the viewpoint of the separator, the basic
requirements for separators in alkaline cells are discussed below and an attempt at
structuring them accordingly is made.
The prime requirements for the separators in alkaline storage batteries are
on the one hand to maintain durably the distance between the electrodes, and
on the other to permit the ionic current flow in as unhindered a manner as
possible. Since the electrolyte participates only indirectly in the electrochemical
reactions, and serves mainly as an ion-transport medium, no excess of electrolyte
is required; that is, the electrodes can be spaced closely together in order not
to suffer unnecessary power loss through additional electrolyte resistance. The
separator is generally flat, without ribs. It has to be sufficiently absorbent, and
it also has to retain the electrolyte by capillary forces. The porosity should be at
a maximum to keep the electrical resistance low (see Section 11.1.2.3); the pore
size is governed by the risk of electronic shorts. For systems where the electrode
substance does not dissolve or is only slightly soluble (e.g., nickel hydroxide,
cadmium), separators which prevent a deposit of particles of the active materials
and subsequent shorting are sufficient, whereas for electrodes that dissolve (e.g.,
zinc), effective ion-selective barriers are desirable, delaying the onset of penetration
from the solution phase. Positive electrodes (e.g., silver oxide) whose ions are
dissolved – even sparingly – and deposited on the negative electrode, form local
elements there, and thus increase self-discharge, also requiring separators with
ion-segregating properties. Ion separation means, however, pore sizes on an atomic