Page 256 - Handbook of Battery Materials

226 8 Metallic Negatives

8.3.7.1 Zinc Electrodes for ‘Acidic’ (Neutral) Primaries
The ‘classical’ Leclanch´ e cell uses zinc sheet formed into a cylindrical can serving
simultaneously as the anode and as the cell container (AB1C1). The cathode is a
mixture of MnO 2 and graphite wrapped into a piece of separator and contacted by
a central carbon rod. The can dissolves slowly when the cell is not in use and faster
when the cell delivers electrical energy.
The reaction following the primary electrochemical zinc dissolution (Equation
8.18) leads, in the case of an ammonium chloride electrolyte, to a zinc diammine
cation:

−
+
Zn 2+ + 2NH + 2OH → [Zn(NH ) 2 ] 2+ + 2H 2 O (8.19)
4
3
The (similar) corrosion reaction is:
+
Zn + 2NH → [Zn(NH ) 2 ] 2+ + H 2 ↑ (8.20)
4 3
If aqueous zinc chloride solution serves as electrolyte (‘heavy duty’ types), the
hydrate of a basic zinc chloride is formed instead of the product in Equation 8.19:
−
5Zn 2+ + 2Cl + 8OH + H 2 O → ZnCl 2 .4ZnO.5H 2 O ↓ (8.21)
−
The construction of the anode (sheet, cell container) is responsible for the two
major problems:
2
−1
• The relatively small specific surface (cm g ) does not allow higher currents;
• The corrosion reactions usually proceed until the can starts leaking and the
electrolyte spillage corrodes the electrical or electronic device powered by the cell.
The corrosion reactions may be slowed down by using zinc alloys (with lead
and cadmium also improving the mechanical properties of zinc to simplify the
production process) instead of the pure metal or by amalgamating the inner surface
of the can by adding a small amount of a mercury compound to the electrolyte.
Only a few studies dealing with corrosion kinetics [122] or with substitutes for
mercury as corrosion inhibitors [123] are available.
Other (‘leak-proof’) cells use an additional steel can on the outside to prevent
any electrolyte loss caused by perforation of the inner zinc beaker when the cell is
exhausted.
The problem of low specific surface (which, however, has a beneficial effect on
the corrosion rate) cannot be solved so easily. This was one important reason for the
development of the alkaline MnO 2 /zinc cell known as ‘Alkaline’or ‘PAM’ (primary
alkaline manganese dioxide).
8.3.7.2 Zinc Electrodes for Alkaline Primaries
The alkaline version of the MnO 2 /zinc cell follows a different concept because it
turns the construction of the Leclanch´ e cell completely around: now the cathode
(MnO 2 + carbon) forms a hollow cylinder contacting the inner wall of the cell
container (steel) along its outer surface. The inner cavity has to accommodate
anode, electrolyte, separator, and current collector. Usually, the separator forms