Page 265 - Lindens Handbook of Batteries

ALKALINE-MANGANESE DIOXIDE BATTERIES 11.3

Positive cover Can

Label
Electrolyte
Anode

Cathode Current collector

Seal
Separator

Negative cover
Neutral cover

FiguRE 11.3 Schematic view of typical alkaline cell construction.
(Courtesy of Energizer, Inc.)

the demand for primary batteries could be limited by competition from rechargeable batteries, such
as the nickel-metal hydride, and the primary 1.5 V lithium chemistries advertised to last up to 7 times
longer than regular alkaline batteries.
Improvements in performance (Fig. 11.2) have resulted in the increased sales shown in Fig. 11.1
due to a combination of improved materials, design, and chemistry. The battery manufacturers have
responded to the higher power and constant current drains required by the new portable devices
being commercialized over the years. When compared to the carbon-zinc system, the alkaline cell
is built inside-out and upside-down, as shown in Fig. 11.3. The positive electrode mix, electrolytic
manganese dioxide, graphite, and potassium hydroxide electrolyte, is molded into a steel can whose
bottom is at the top of the pictured cutaway. A paper separator basket or two strips are inserted, and
a potassium hydroxide gel containing powdered zinc is dispensed into the basket. The electrolyte
also includes an inhibitor to mitigate zinc corrosion and ensure long shelf life. A negative collector
assembly consisting of a brass nail and a plastic seal are inserted, making contact with the zinc gel.
A flat cover is then placed over the open part of the can and crimped shut, becoming the negative
end of the cell. The steel can bottom, the positive contact, is also provided with a cover, sometimes
having a center dimple, which forms the positive end of the finished cell.
During the evolution of the alkaline cell over the past 50 years, many improvements have been
made to this cell design. After the initial concept of using a gelled zinc powder anode and the use
of a vented plastic seal, the first major advance was the butt-seam metal finish, which provided an
increased internal volume. This was followed by the discovery that the addition of organic inhibitors
to the anode could reduce the rate of gassing caused by impurities or contaminants in the zinc anode,
which resulted in a product with bulge and leakage problems. Another major development was the
introduction of a plastic label and lower profile seal, which even further increased the cell’s internal
volume that allowed the addition of more active materials and thus a greater discharge capacity. One
of the most important developments of the alkaline cell in the 1980s was the gradual reduction in the
amount of added mercury in the anode. Most early alkaline cells contained up to 6% mercury in the
zinc anode, but with the development of cathode materials with lower impurities and better process-
ing techniques, the level of added mercury was gradually reduced to zero. This objective of removing
all added mercury was driven by the worldwide concern over the environmental impact of the cell
components after their disposal. Today, most countries have banned batteries that contain mercury.