An ancient natural red plant dye could be used to create an eco-friendly, sustainable, lithium-ion battery, researchers say.
The dye the researchers are investigating for use in batteries comes from the roots of a climbing herb known as the madder plant. Civilizations in Asia and the Middle East first boiled madder roots more than 3,500 years ago to extract purpurin, a nontoxic dye they used to color fabrics vivid oranges, reds and pinks.
"This is a paradigm shift. For the last 3,000 years, we've used purpurin as a dye material — we never thought this could be used in energy storage," researcher George John, an organic chemist at the City College of New York, told TechNewsDaily. "This is definitely exciting to us and maybe future generations."
Rechargeable lithium-ion batteries power mobile devices and electric vehicles. Most lithium-ion batteries rely on cobalt as a key ingredient in the electrodes that electric current flows through.
"Thirty percent of globally produced cobalt is fed into battery technology," said researcher Arava Leela Mohana Reddy, a materials scientist at Rice University.
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However, mining cobalt is expensive, and supplies are finite. Fabricating and recycling lithium-ion batteries made with cobalt also demands high temperatures and lots of energy. In 2010, almost 10 billion lithium-ion batteries had to be recycled, Reddy said.
Moreover, producing and recycling the batteries pumps an estimated 158 pounds (72 kilograms) of the greenhouse gas carbon dioxide into the atmosphere for every kilowatt-hour of energy in the batteries. All these factors have driven research into developing greener batteries using biologically based molecules, Reddy said.
After investigating a number of molecules from nature, the researchers found that purpurin and its relatives seem ideal as electrode materials. For instance, purpurin has electron-rich molecular rings adept at passing electrons back and forth just as traditional electrodes do, easily coordinating with lithium.
In addition, the researchers found they could make purpurin into an electrode in only a few easy steps, all relatively cheaply at room temperature — they simply dissolved the dye in an alcohol solvent and added lithium salt. When the salt's lithium ion binds with purpurin, the solution turns from reddish yellow to pink.
Furthermore, growing madder would soak up carbon dioxide, not generate the global warming gas. The way purpurin binds to lithium renders it nontoxic as well, which suggests lithium-ion batteries made with it could just be thrown away without the need for costly recycling.
"Purpurin comes from nature and it will go back to nature," John said.
The electrical performance of the battery they devised using purpurin "is within the ballpark of most conventional lithium-ion batteries," Reddy told TechNewsDaily. For comparison, if a conventional lithium-ion battery had a performance score of 140, their battery performed at roughly 100, he said.
One weakness of the device is that the electrolyte material that lithium-ion battery electrodes are normally immersed in breaks down purpurin-based electrodes. "However, we are very confident we can come up with a new electrolyte for a stable green lithium-ion battery," Reddy said.
Future research will aim to improve purpurin's efficiency through chemical tinkering and to develop similar molecules that might perform even better than purpurin.
"We are now looking for a funding support to explore a few more biological-molecule-based systems and industry collaborators so we can go to the next level with this technology," Reddy said.
The scientists detailed their findings online Dec. 11 in the journal Scientific Reports.
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