Page 266 - Lindens Handbook of Batteries

11.4 pRIMARy BATTERIES

TABLE 11.2 Comparison of Advantages and Disadvantages of Miniature
Alkaline-Manganese Dioxide Cell to Other Miniature Systems
Advantages Disadvantages
Lower cost Sloping discharge
Lower internal resistance Lower energy density
Good low-temperature performance Shorter shelf life
Equivalent leakage resistance

These improvements in materials and construction have allowed the alkaline-MnO battery to gain
2
as much as a 60 to 70% increase in specific energy output since its first introduction. These improve-
ments have allowed alkaline batteries to keep pace with the needs of consumers and their demand
for smaller and higher energy devices. The continuing research by the major battery companies will
provide further technological improvements to ensure their leadership roles in the market.
As already mentioned, the miniature alkaline cell uses the same zinc/alkaline electrolyte/manga-
nese dioxide configuration as the cylindrical cells. It competes with the other miniature cell battery
systems, such as silver oxide and zinc/air as well as lithium-based chemistries. The major uses of
this cell configuration are in watches, hearing aids, and specialty items. This cell consists of a shal-
low steel can that holds the cathode and serves as the positive contact and a copper-clad steel cover
containing a potassium hydroxide gel with zinc powder that is the negative contact. Table 11.2 lists
the advantages and disadvantages of the alkaline-manganese dioxide miniature battery compared to
other lithium-based miniature systems.

11.2 CHEmISTRy

The active components of the alkaline-manganese dioxide cell include powdered zinc, an aqueous
KOH electrolyte, and electrolytically produced manganese dioxide. The electrolytic MnO or EMD
2
is used instead of either chemical MnO or the natural ore because of its higher manganese content,
2
increased activity, and higher purity. The KOH electrolyte is a high-purity concentrated solution
typically in the range of 35 to 52%, which provides a high conductivity and reduced gassing rate
for the sealed alkaline cells for the various device applications and storage conditions. The zinc
powder anode provides a high surface area for high-rate capability, i.e, low local current density, and
facilitates the homogeneous distribution of the solid and liquid phases in the anode compartment to
minimize concentration polarization of the reactants and products.
During discharge, the manganese dioxide cathode first undergoes a one-electron reduction to the
oxyhydroxide with expansion and distortion of its lattice in concentrated alkaline electrolytes.
-
MnO + H O + e → MnOOH + OH (11.1)
2
2
The MnOOH product forms a solid solution with the reactant, which produces the characteristic
sloping discharge. Of the many structural forms of MnO that exist, only the gamma form has the
1
2
best alkaline discharge characteristics, because its surface is not prone to being blocked by the reac-
tion product. Manganese dioxide has been identified to have at least nine crystal structures, one of
which is the gamma form, known in nature as nsutite, an intergrowth of the beta or pyrolusite form
and ramsdellite. It is this structurally disordered form of manganese dioxide that is found in alkaline
cells. It is composed of the 1 × 1 tunnel structure of pyrolusite and the 1 × 2 tunnel form of the rams-
dellite, as depicted in Fig. 11.4. Table 11.3 shows the different manganese oxide structures. 3
2
The cathode expands about 17% in volume when forming the MnOOH reaction product. MnOOH
can also undergo some undesirable chemical side-reactions, depending on the conditions and extent
of the discharge. In the presence of the zincate ion, MnOOH, based on its equilibrium with soluble
Mn(III), can form the complex compound hetaerolite, ZnMn O . Although electroactive, hetaerolite
2
4
is not as easily discharged as MnOOH, and results in an increased cell impedance. In addition, the