Separator Materials for Alkaline Batteries

In the product range of alkaline power sources each manufacturer has developed 

Of all possible manufacturing proceses for macroporous separators to be employed in alkaline batteries, the wet-fleece process using paper machines is the predominant one [130] it permits a very uniform ( cloud-free ) production of such material and the use of different types of fibers as well as of short and very thin fibers, thus achieving a uniform structure of small pores (Table 15). 

Whereas PVA fleeces are used only in primary cells polyamide fleeces compete with polyolefin, preferably polypropylene fleeces. The latter are more stable at higher temperatures and do not contribute to electrolyte carbonation, but they wet only after a pretreatment either by fluorination [131] or by coating and crosslinking with hydrophilic substances (e.g., polyacrylic acid [132]) on the surface of the fiber. 

Only very recently the production of melt-blown polypropylene fleeces with considerably thinner fiber diameter became possible [100], thus making it possible — a low-cost hydrophilization provided — to achieve attractive properties with regard to small pore size and excellent tensile performance for use in highly automated assembly processes. 

Very different microporous separators for alkaline batteries are included in Table 16. The very thin (-25 ftm) films of stretched polypropylene ( Celgard ) are generally employed in combination with 

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Most commonly used separator materials for alkaline Zn/Mn02 batteries are nonwoven polymers, such as cellulose, vinyl polymers, polyolefin, and others. The separator materials must be chemically stable in concentrated KOH solutions and electrochemically stable under both oxidizing and reducing conditions in the cell. In addition to its good electronic insulation, physical strength, and porous structure, good wettability to concentrated KOH solutions is especially crucial to provide a good ionic pathway for the battery operation. 

Nonwovens have been well established as traditional materials for alkaline batteries, NiCd and NiMH cells, and for absorbed glass matt (AGM) lead—acid batteries. Alkaline batteries use a variety of materials, mostly composed of wet-laid cellulose and polyvinyl alcohol (PVA). NiCd and NiMH use a variety of materials, from spunbond and melt blown to dry-laid materials. Lead—acid AGM batteries use a wet-laid glass matt as the separator, often in conjunction with another membrane. These materials range in thickness from 100 to 300 pm. One review article in 2007 even states that for lithium ion cells, nonwoven not possible. However, these traditional nonwovens are a significant industrial market for several manufacturers, with total sales into these types of cells of 450 million globally. 

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

Cylindrical alkaline cells are made in only a few standard si2es and have only one important chemistry. In contrast, miniature alkaline cells are made in a large number of different si2es, using many different chemical systems. Whereas the cylindrical alkaline batteries are multipurpose batteries, used for a wide variety of devices under a variety of discharge conditions, miniature alkaline batteries are highly speciali2ed, with the cathode material, separator type, and electrolyte all chosen to match the particular appHcation. 

A separator was not required for this kind of battery. Average coulombic and energy efficiencies were up to 96% and 86%, respectively [57]. Over 1,000 cycles were achieved in a small laboratory cell. Zinc deposition from an alkaline zincate solution has been also investigated [58], and results show that zinc deposition on the cadmium substrate is better than other materials, for example, Cu, Pb. On the cadmium-coating electrode, electroplating behavior was simple and facilitated the discharge of zinc. 

The separator must serve this purpose in a dynamic environment. Electrodes can change shape and/or volume as a function of age and cycle number electrolytes can age triggering precipitation. The separator must be considered and matched to its system so that it can provide the aforementioned functionality for the design life of the system. Figure 2 indicates the workhorse design of perhaps the most ubiquitous battery and separator material, the zinc-alkaline primary system. 

Electrochemical Design. The electrochemical design of the cell consists primarily of balancing the active materials present in the electrodes. This previously has been discussed for each of the two electrodes separately, the nickel positive electrode and the zinc negative electrode. When combined in the cell, the two active materials must be present in some ratio with respect to each other. As with most other alkaline nickel batteries, the nickel-zinc system is typically positive (nickel electrode) limited. This means that the cell contains more zinc active material, on an Ampere-hour basis, than nickel active material. This must take into account the active materials present in the cell in addition to the active material utilization of each. 

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