The quest for a way to create a rechargeable battery from sodium rather than lithium took a somewhat unexpected turn last month when scientists from the University of Maryland and the National Center for Nanoscience and Technology from Beijing discovered that a baked oak leaf pumped full of sodium made a successful negative terminal for a proof-of-concept battery.

As anyone who ever made a battery from a lemon (or a potato) would remember, making a battery out of organic matter is certainly possible and could play its part in developing a workable sodium-based battery.

Sodium theoretically should hold more charge than lithium, but in practice it can't handle as many charge-and-discharge cycles. One of the stumbling blocks has been finding a suitable anode material that is compatible with sodium. Graphite, which is used in Li-ion batteries, is not suitable since sodium ions have a much larger ionic size than lithium. Graphene is a possibility, but is time consuming and expensive to produce.

Earlier experiments have included other biomass sources, such as peat moss, banana peels, and melon skin, but these require additional processing and coating.

"The natural shape of a leaf already matches a battery's needs: a low surface area, which decreases defects; a lot of small structures packed closely together, which maximizes space; and internal structures of the right size and shape to be used with sodium electrolyte," said Fei Shen, a visiting student working on the project.

The scientists first processed a dry leaf through pyrolysis, baking it at 1,000° C (1,832° F) for one hour in an oxygen-deprived medium to burn off all but the underlying carbon structure. To remove other inorganic impurities that might interfere with the electrochemical processes, they immersed the leaf for 6 hours in hydrogen chloride.

The result was a carbonized leaf, still studded with the pores on the underside that usually allow the leaf to absorb water and exchange gases. In the leaf's carbonized state, the pores were more than adequate for absorbing a sodium electrolyte. The layers of carbon on the tougher topside of the leaf become sheets of nanostructured carbon that absorb the sodium that carries the charge. In its carbonized state, the topside of the leaf is flat, dense and suitable as a current-collecting plane during electrochemical reactions.

The team used coin batteries with sodium plates as the counter/reference electrodes, and the leaf proved itself more than adequate as the anode of a sodium battery, capable of storing 360 mAh per gram of its weight.

The leaf anode was tested for cycling and proved stable by holding 90 percent capacity after 200 cycles. Interestingly, the charge efficiency also remained relatively high at around 75 percent, which the scientists attribute to low SEI (Solid Electrolyte Interphase) formation thanks to the low surface area of the leaf-membrane.

"We have tried other natural materials, such as wood fiber, to make a battery," said Liangbing Hu, an assistant professor of materials science and engineering and a researcher with the University of Maryland Energy Research Center (UMERC). "A leaf is designed by nature to store energy for later use, and using leaves in this way could make large-scale storage environmentally friendly."

The researchers have experimented with stacking multiple leaves welded together and plan on testing different types of leaves to find the ones with best properties for energy storage. They say they have no immediate plans to commercialize the idea.

They published their findings in January through ACS Applied Materials & Interfaces.

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