They've been used as a rubber additive, a carbon-capture medium and a bioplastic ingredient. Now, discarded eggshells may have yet another use. Scientists have determined that they could be utilized in an eco-friendly and inexpensive form of energy storage.

An international team of researchers started with regular chicken eggshells, which they washed, dried and crushed into a fine powder. That powder consisted both of the calcium carbonate outer shell, along with the protein-rich fiber membrane found inside.

The material was then used as an electrode against a metallic lithium anode, within a non-aqueous electrolyte. When tested, the resulting battery cell was found to maintain a capacity retention of 92 percent, over the course of more than 1,000 charge/discharge cycles (capacity retention is defined as "a measure of the ability of a battery to retain stored energy during an extended open-circuit rest period").

This energy-storage capability came thanks mainly to the calcium carbonate, which allowed the crushed egg shells to store lithium. It is now hoped that the material could be incorporated into a low-cost lithium-ion capacitor.

"Surprisingly, there are constantly new examples in which natural substances are found to be suited well or even very well for producing materials for electrochemical storage systems," says Prof. Maximilian Fichtner, of Germany's Helmholtz Institute Ulm.

A paper on the research was recently published in the journal Dalton Transactions.