Page 210 - Lindens Handbook of Batteries

9.4 PRIMARY BATTERIES

9.2 CHEMISTRY

The zinc-carbon cell uses a zinc anode, a manganese dioxide cathode, and an electrolyte of ammo-
nium chloride and/or zinc chloride dissolved in water. Carbon (acetylene black) is mixed with the
manganese dioxide to improve conductivity and retain moisture. As the cell is discharged, the zinc
is oxidized and the manganese dioxide is reduced. A simplified overall cell reaction is
Zn + 2MnO → ZnO · Mn O
2 2 3
In actual practice, the chemical processes that occur in the Leclanché cell are significantly more
complicated. Despite the 125 years of its existence, controversy over the details of the electrode
7
reactions continues. A chemical “recuperation” reaction may operate simultaneously with the dis-
charge reactions. 5
This could result in several intermediate states that confuse the reaction mechanisms. Furthermore,
the chemistry is complex because it is a non-stoichiometric oxide and is more accurately represented
as MnO , where x typically equals 1.9+. The efficiency of the chemical reaction depends on such
x
things as electrolyte concentration, cell geometry, discharge rate, discharge temperature, depth of
discharge, diffusion rates, and type of MnO used. A more comprehensive description of the cell
2
reaction is as follows: 4
1. For cells with ammonium chloride as the primary electrolyte:
Light discharge: Zn + 2MnO + 2NH Cl → 2MnOOH + Zn(NH ) Cl
2 4 3 2 2
Heavy discharge: Zn + 2MnO + NH Cl + H O → 2MnOOH + NH + Zn(OH)Cl
2 4 2 3
Prolonged discharge: Zn + 6MnOOH → 2Mn O + ZnO + 3H O
3 4 2
2. For cells with zinc chloride as the primary electrolyte:*
Light or heavy discharge: Zn + 2MnO + 2H O + ZnCl → 2MnOOH + 2Zn(OH)Cl
2 2 2
or: 4Zn + 8MnO + 9H O + ZnCl → 8MnOOH + ZnCl ⋅ 4ZnO ⋅ 5H O
2 2 2 2 2
Prolonged discharge: Zn + 6MnOOH + 2Zn(OH)Cl → 2Mn O + ZnCl ⋅ 2ZnO ⋅ 4H O
3 4 2 2
In the theoretical case, as discussed in Chap. 1, the specific capacity calculates to 224 Ah/kg,
based on Zn and MnO and the simplified cell reaction. On a more practical basis, the electrolyte,
2
carbon black, and water are ingredients that cannot be omitted from the system. If typical quantities
of these materials are added to the “theoretical” cell, a specific capacity of 96 Ah/kg is calculated.
This is the highest specific capacity a general-purpose cell can have and is, in fact, approached by
some of the larger Leclanché cells under certain discharge conditions. The actual specific capacity of
a practical cell, considering all the cell components and the efficiency of discharge, can range from
75 Ah/kg on very light loads to 35 Ah/kg on heavy-duty, intermittent discharge conditions.

9.3 TYPES OF CELLS AND BATTERIES

During the last 125 years, the development of the zinc-carbon battery has been marked by gradual
change in the approach to improve its performance. It now appears that zinc-carbon batteries are
entering a transitional phase. While miniaturization in the electrical and electronic industries has
reduced power demands, it has been offset by the addition of new features requiring high power, such
as motors to drive compact disc players or cassette recorders, halogen bulbs in lighting devices, etc.
This has increased the need for a battery that can meet heavy discharge requirements. For this reason,

*Note: 2MnOOH is sometimes written as Mn O ⋅ H O and Mn O as MnO ⋅ Mn O . Electrochemical discharge of MnOOH
3
2
4
2
3
3
2
vs. zinc (prolonged discharge) does not provide a useful operating voltage for typical applications.