When the lead storage battery is being used as a power source, oxidation-reduction reactions generate electricity.

When a depleted battery is being recharged, electricity from an outside source causes oxidation-reduction reactions to go against the natural free energy gradient.

This same principle of using electric current to bring about energetically unfavorable chemical changes is used widely in electrolytic cells.

Many metals can be obtained from their ores (usually oxides or sulfides) by reducing them with carbon, but the alkali metals, such as sodium, are too reactive for this.

They must be obtained by electrolysis.

In the electrolysis of molten sodium chloride, the reduction that cannot be accomplished chemically is carried out electrochemically. Current is passed through two electrodes immersed in NaCl heated above its melting point of 801⁰C.

At one electrode, Cl^- ions are oxidized to Cl₂ gas, and it therefore is the anode. At the cathode, Na⁺ ions are reduced to sodium metal.

The Cl₂ gas is collected and piped away, and the sodium, which is a liquid at this temperature and lighter than the fused salt, floats to the surface and is recovered.