Nanoporous silicon is believed by many to be the next-generation anode material in lithium-ion batteries as it offers the benefit of a much higher theoretical capacity and lower operating voltage than the commonly used graphitic carbon materials. The big challenge, however, is finding a suitable low-cost strategy to employ an appropriate nanostructured silicon material that would compensate for the large volume expansion upon lithium insertion.
A truly distinct approach from the elaborate physical or chemical treatments of expensive silicon precursors has been explored by Yan Yu and her colleagues at the Max Planck Institute for Solid State Research, the University of Science and Technology of China, and the South China University of Technology. The scientists imagined that the hierarchical architecture of the silica already present in the leaves of common reed plants can readily transform into the micro- and nanoporous silicon architecture required for lithium-ion battery materials.
"Reed leaves exhibit well-defined sheetlike 3D hierarchical microstructures," they point out, "which can be transformed into the 3D highly porous hierarchical silicon architectures by magnesiothermic reduction."
This magnesiothermic reduction was also combined with a simple carbon-coating step to eventually achieve an anodic material exhibiting high specific capacity, very good rate capability, and cycling stability, just as it is required in advanced lithium-ion batteries.
What is particularly interesting here is that the topological architecture of the original silicates within the reed leaves is extraordinarily well preserved during the applied chemical and physical treatment steps. Upon the purification from the dry reed leaves, the three-dimensional structure only shrinks, but retains its mesoporous network. It does not even change during the reduction to the final carbonized silicon network. This robustness in structure preservation makes reed plants, which grow as large monocultures in the wetlands of temperate regions, especially suitable as a new, sustainable feedstock for battery materials.