R16 A zinc battery and a button cell

Illustration R16 shows two different batteries which are in everyday use. Their operation can be compared with that of the lead-acid accumulator (see illustration R15).

Zinc battery

This battery, which has a rather unusual construction, still carries the name of its inventor, the French scientist Leclanché. It is the basis of the familiar 1.5 V batteries used in torches and other household appliances requiring batteries. The cathode consists of a central current collector (which can for example be made of copper) surrounded by a paste of MnO₂ , KOH, ZnO and small graphite particles. The graphite particles, which are present in high concentration, are there to conduct electrons to the central collector. The anode, which forms the shell of the battery, is made of zinc. During discharge of the battery this zinc layer is slowly oxidised to zinc oxide, whilst in the cathode compartment Mn(IV) ions in MnO₂ are reduced at the surface of the graphite particles.
The two electrode-compartments are separated by waxed paper and/or wax.

The half-cell-reactions for the Leclanché cell are:

Zinc (the anode, at the top right of the table of the standard reduction potentials in illustration R7) is the reducing agent, being oxidised to zinc ions.
The oxidising agent is the MnO₂ (in the cathode compartment) which is found at the bottom left of a standard table of reduction potentials. Mn(IV) ions are reduced during discharge, its oxidation number being changed from +IV to +III.

The standard reduction potentials for the two half-reactions can be used to calculate the standard reduction potential for the cell:

In practice the cell potential is 1.55 V, the difference being due to non-standard concentration of ions in the cell (see illustration R8).

This cell is non-rechargeable due to the following irreversible reactions :

During the last few years, rechargeable batteries have been developed using a similar construction by preventing these irreversible reactions from taking place.

Button cells.

Button cells, which are used in watches, cameras, calculators etc., work on a similar principle. The half-cell-reactions are:

The reducing agent (anode) is once again metallic zinc.
The oxidising agent, which is contained in the cathode compartment, is silver(I) oxide mixed with graphite particles, mercury(II) oxide, and potassium hydroxide (the electrolyte). The overall discharge reaction is:

These batteries are small and therefore only have a small capacity.