Next-generation high-capacity lithium-ion batteries could open doors to better electric vehicles and a multitude of other energy storage applications. To increase battery capacity, many researchers want to replace the carbon anode in today’s technology with a silicon anode that has the potential for ten times the capacity.

So far, attempts to design silicon anodes have struggled. Capacity drops off quickly with each battery cycle because the silicon swells as it charges and shrinks as it discharges, which can cause fracturing. Many researchers are seeking nano-structural solutions that will accommodate the material’s expansion, but a team of Korean scientists turned to what may be the most surprising source for high-tech electronics components—rice.

The husks of the rice grains, which are discarded as waste after harvest, are about 20 percent silica. The silica forms a layer that prevents attack from insects and bacteria but is porous enough to allow the transfer of air and water. Dae Soo Junga and team wanted to see if that naturally occurring nanostructure could solve the capacity fade problems that plague silicon anodes.

After acid and heat treatments that remove organic compounds and metals, the rice husks are more than 99.9 percent silica. The natural nanostructure of the silica survives these treatments intact.

So far, so good. The tricky part is converting the rice’s porous silica structure into silicon. Silica is silicon bonded to two oxygen atoms, which need to be removed if the material could be used as an anode. The researchers tried a reduction technique to remove the oxygen using magnesium and really high heat. They found that this process alters the nanostructure slightly, creating silicon walls but retaining the pores.

Before testing the new silicon structure as an anode, they coated it in carbon layers to increase conductivity. The scientists found that this new material retained its full capacity after 200 cycles and showed stronger performance than carbon-coated silicon nanoparticles, suggesting that the porous structure gives the silicon the space it needs to expand and contract without deforming.

The authors suggest that rice husks could be recycled in the future to create these anodes at a production scale. Whether this process could be scaled up in a cost-effective manner is still an open question, but as an example of what engineers can learn from nature’s highly evolved nanostructures, it’s pretty cool.

PNAS, 2013. DOI: 10.1073/pnas.1305025110 (About DOIs.)