Page 270 - Lindens Handbook of Batteries

11.8 pRIMARy BATTERIES

to almost boiling. The typical electrolysis cell of most EMD manufacturers consists of a tita-
nium anode and copper cathode. However, in the past, lead and graphite cathodes have also been
employed. The anode reaction to deposit solid MnO alkaline cell product is as follows:
2
2+
+
Mn + 2H O → MnO + 4H + 2e (11.9)
2
2
Hydrogen is formed at the cathode according to Reaction (11.10):
+
2H + 2e → H ↑ (11.10)
2
Thus the overall reaction for the plating of EMD is
MnSO + 2H O → MnO + H SO + H (11.11)
4 2 2 2 4 2
The important plating variables to produce an EMD suitable for use in alkaline cells require
precise control of the bath’s temperature, current density, and component concentrations. Once the
EMD is removed from the anode, it is crushed, washed, ground, and dried. Each battery manufac-
turer has its own EMD specification, so one type does not fill all requirements.
The analysis of a typical EMD is shown in Table 11.5. The low levels of impurities are essential
in order to minimize the hydrogen gassing at the anode if these elements become soluble and diffuse
to the anode. The other listed parameters are also important, and their listed ranges all go toward
providing an EMD suitable for alkaline cell use.
Other important EMD characteristics include its surface area and hardness. The surface area,
dictated by the porosity and particle size distribution, determines the current density in the cathode,
which is especially important for high-rate discharge applications. EMD is typically a very hard
material, and this hardness affects the milling of the EMD and tool wear of the equipment used to
make the cathode mixes and molding equipment. premature tool and mill wear can introduce iron
impurities into the pure EMD and add cost to the overall battery manufacturing process.

Carbon. EMD is a relatively poor conductor in its undischarged state and even worse when par-
tially discharged. To overcome this problem, carbon, typically in the form of graphite, is added to the
cathode mix to enhance its overall electronic conductivity. The graphite provides a conductive matrix
so the electrons can be evenly distributed throughout the cathode, thus lowering the overall current
density in the cathode. However, one must strike a balance between the amount of added carbon
and the EMD level. Additional carbon provides a more conductive cathode matrix, but it reduces
the amount of active material in the cathode. Therefore, the ratio of carbon to EMD in the cathode
needs to be optimized for the required applications of the battery. Over the years, many changes

TABLE 11.5 Typical Analysis of Electrolytic Manganese Dioxide (EMD)
Component Typical Value* Component Typical Value*
MnO content >91% Ti <5 ppm
2
Mn >60% Cr <7 ppm
peroxidation >95% Ni <4 ppm
H O, 120C <1.50% Co <2 ppm
2
H O, 120–400C >3.0% Cu <4 ppm
2
Real density 4.45 g/cm 3 V <2 ppm
K <300 ppm Mo <1 ppm
Na <4000 ppm As <1 ppm
Mg <500 ppm Sb <1 ppm
Fe <100 ppm pb <100 ppm
C 0.07% SO 2- ≤0.85%
4
*Based on analyses of typical alkaline-grade EMD.