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what is spongy lead?

A spongy lead or soft metal is a form of metallic lead (Pb) that is in a cellular structure with high porosities- 75-95% of the volume is empty space. These characteristics are a result of the metal’s ability to absorb energy and conduct it away from the surface, making it useful for a variety of applications including heat exchangers, energy absorption, flow diffusion, lightweight optics, etc.

The sponge lead was invented in the early 19th century by Charles Francis Brush, an inventor of lead-acid batteries. He made a ribbed or grooved sheet of lead, which was filled with pulverized lead oxychloride and placed horizontally in a salt or acid solution.

He then electrolytically reduced this lead oxychloride to sponge lead, forming a functioning cell electrode. The spongy lead was then converted back to lead oxychloride upon discharge, or to pure lead in a spongy form when the cell was recharging.

In a lead acid battery, the positive plate is lead peroxide and the negative plate is sponge lead in the soft, porous form. These plates are dipped into diluted sulfuric acid (H2SO4) to allow a chemical reaction between the lead and acid to occur.

The dissolved acid splits into positive hydrogen ions and negative sulfate ions. These ions react with the lead in the sponge lead plate, forming lead dioxide and lead sulfate. This leads to a chemical reaction between the lead and sulfuric acid which produces electricity. The battery can then be charged by reversing the process and recharging with the same diluted sulfuric acid.