Page 179 - Lindens Handbook of Batteries

7.4 PRINCIPLES OF OPERATION

dendritic and mossy growth, it is inevitable that separator bridging and shorting will occur to limit
the cycle life of the cell. Most commercial rechargeable zinc batteries have focused on additives to
the KOH electrolyte to try to improve the morphology of the deposit, and reference is made to the
individual chapters of this book for information.
The electrolyte for primary alkaline batteries usually has a gel-forming polymer added to the
solution, although the earlier wet cells such as Zn-air and Zn-CuO cells used the liquid without gela-
tion. The gel-forming polymer must of course be alkali stable. For many years, the favorite polymer
was sodium carboxymethylcellulose, first used in the zinc-mercuric oxide primary cell to immobilize
the electrolyte and convert it from a wet cell to a dry cell. This material is still used in some alkaline
batteries, but is subject to oxidation by high potential cathodes. Polymers such as other cellulosic or
starch derivatives, polyacrylates, or ethylene maleic anhydride copolymers are used in some alkaline
batteries. The manufacturers generally keep their anode gel formulation as trade secrets, so not much
information can be included in this section. An effect of the use of gelling agents is to lower the
conductivity of the electrolyte phase and this must be taken into account in the cell design so that
electrolyte path lengths are not too long. It should be mentioned that zero-added mercury cells have
become predominant, especially in alkaline-MnO cells, thus placing greater importance on electro-
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lyte purity, particularly with regard to iron and chloride ion since these materials are corrosion acce-
lerants. Of course, all of the battery materials are required to have good purity with regard to these
and other elements that can accelerate corrosion. Group 3 and Group 5 elements such as indium
and bismuth have taken the place of mercury in many alkaline types in order to reduce the landfill
burden of this toxic element. In addition, certain organic additives, which have corrosion inhibition
properties, are included in the electrolyte formulation. In fact, the use of mercuric oxide cells has
declined substantially due to this concern, although the high-voltage stability of the Zn-HgO and
Cd-HgO batteries has preserved some high-end electronic applications for these systems. Notably,
the Cd-HgO battery can be regarded as one of the few electrolyte stable aqueous batteries with its
operating voltage of about 0.9 V. This factor has permitted its use even in high-temperature applica-
tions where a zinc anode would corrode substantially.
Alkaline electrolytes are very important for rechargeable batteries and are frequently used in
specially designed wet cells. The overpotential during charging can cause oxygen gassing with metal
oxide positive electrodes, which is very deleterious to polymer gelling agents. For example with
nickel oxyhydroxide positive electrodes, the desire to get high cycle-life mitigates against using most
organic compounds in the electrolyte. The exception is for Zn-MnO rechargeable cells (see Chap. 28)
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which have limited cycle life for other reasons and thus the oxidation of gelling agents, which is
relatively slow, is not the limiting problem. In fact, the anode gel in this rechargeable cell is similar
to that of the primary alkaline battery. Additives are also an important aspect of alkaline rechargeable
cells. Lithium hydroxide is a common additive for nickel cadmium batteries, and sometimes cobalt
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salts are incorporated in the electrolyte for cells with nickel oxyhydroxide positive electrodes. The
history of nickel-zinc secondary cells is essentially that of various electrolyte additives, the main
purpose of which is to suppress the solubility of zinc in the alkaline electrolyte. In the absence of
such additives, the formation of zinc dendrites on charge severely limits the cycle life of the cell.
These topics are dealt with in Chap. 23.

7.2.2 Neutral Electrolytes
The main battery type that uses neutral or slightly acidic electrolytes is the Leclanché or carbon-
zinc battery. There are two main versions of this cell, namely the Leclanché electrolyte, which is a
combination of zinc chloride and ammonium chloride salts dissolved in water, and the zinc chloride
cell, which is mainly zinc chloride dissolved in water with a small amount of ammonium chloride
sometimes added. A typical Leclanché electrolyte contains about 26% NH Cl, 9% ZnCl , and 65%
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water, while a typical zinc chloride electrolyte contains about 30–40% ZnCl and 60–70% water. A
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small amount of corrosion inhibitor is also usually contained in the electrolyte for ultimate deposi-
tion on the zinc surface. The conductivity of zinc chloride solutions reaches a maximum of 0.107 S/cm
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at 3.7 M ZnCl with a relatively slow decrease with increasing or decreasing concentrations.
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