Separators for alkaline storage batteries

Batteries with gelled electrolyte have been shown to require a separator in the conventional sense, to secure spacing of the electrodes as well as to prevent any electronic shorts the latter is achieved by microporous separators. An additional important criterion is minimal acid displacement, since these batteries — in comparison with batteries with liquid electrolyte — lack the electrolyte volume share taken up by gelling and by the cracks. 

Among the separator varieties described, the phenol-formaldehyde-resorcinol separator (DARAK 2000) [60] as well as the microporous PVC separator [86] have proven effective for this construction. For applications without deep discharges, concessions may be made with the respect to porosity and pore sizes of the separator therefore polyethylene separators or a spe- 

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 medium 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 1.15 - 1,45 gem 3. Several elec- 

Supplier Daramic, Inc. [60, 61 ] AMER-SIL s.a. [86] Lydall Axohm [73] AMERACE Microporous Products [87] 

Brand name Darak 2003 lnd. with glassmat DGT 200 HP Standard D. S. R. Micropor-Sil 

For alkaline storage batteries requirements are often demanded exceeding 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, 108, 109]. 

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. 

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Separators for Alkaline Storage Batteries JJ5 Table 11.15 Nonwoven materials for alkaline batteries. 

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 ion-transport medium, no excess of electrolyte is required, i.e., 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 Sec. 9.1.2.3) the pore size is governed by the risk of electronic shorts. For systems where the electrode substance... 

The development of stable, effectively ion-separating membranes for rechargeable alkaline batteries remains a persistent challenge, in which the separator can provide decisive contributions to the advancement of storage batteries of high power and energy density. 

Nickel/metal hydride (Ni/MH) battery is a secondary battery using hydrogen storage alloy for the negative electrode, Ni(OH)2 for the positive electrode, and alkaline solution for the electrolyte. Polypropylene nonwoven fabric is usually selected for the separator. The theoretical voltage is about 1.32 V, and the operating voltage is about 1.2 V which is almost the same as that of Ni/Cd battery [1]. The Ni/MH battery has been put to practical use for portable electric equipments in 1990 and for HEV (hybrid electric vehicle) in 1997 [2, 3]. 

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