Now, rice has made it onto this list. Researchers in Korea have found that rice husks – the outer, protective covering of a rice kernel – can be a source of silicon that can be used for high-capacity lithium battery anodes.
Most of today's lithium-ion batteries rely on anodes made from graphite, a form of carbon. There are several candidate electrodes to replace graphite as the anode for lithium-ion batteries and silicon has been recognized as a favorable anode material because its capacity is 3-5 times larger than those of existing graphite anodes.
However, so far the cycle life performance of silicon anodes has been challenging due to large volume expansions. The problem has been that previous silicon anode designs broke down from repeated swelling and shrinking during charging/discharging cycles and quickly became useless.
The new work, reported in the July 8, 2013 online edition of PNAS ("Recycling rice husks for high-capacity lithium battery anodes"), demonstrates that rice husks, a major by-product in rice harvest, can be used to produce silicon with an ideal porous nanostructure for use in high-capacity lithium-ion battery (LIB) anodes.
"The interconnected nanoporous structure of rice husks, developed in years of natural evolution for efficient cultivation of rice, can resolve important issues in silicon anode operation, enabling excellent cycling and power performance," Jang Wook Choi, an Associate Professor at the Korea Advanced Institute of Science and Technology (KAIST), who led the work, tells Nanowerk. "Given that annual global rice production reaches 700 hundred million tons, the promising battery data herein show how a part of the waste from rice husks – which reaches 20 wt% of the entire rice kernel – can be a massive resource to meet the ever-increasing demand for silicon in advanced batteries."
In this new research, Choi's team found a naturally existing nanoporous structure in rice husks, with the pores the size of tens of nanometers. Various purification treatments of ordinary rice husks, which preserved the unique 3D porous structure, resulted in over 99% pure silica. A further thermic reduction then transferred the 3D porous structure of the rice husk silica to silicon.
To evaluate the rice husk silicon as a LIB anode, the researchers prepared coin-type half-cells.
"For this testing, we coated the rice husk silicon with carbon layers (3-10 nm) to compensate for the intrinsically low electronic conductivity of silicon as shown in previous studies," explains Choi. "We found that this carbon-coated rice husk silicon anode material shows excellent discharge capacity retention during cycling using the stable interconnected porous structure. Its cycling performance is clearly better than those of bare silicon nanoparticles and carbonized polydopamine-coated silicon nanoparticles."
These results lead him to conclude that the nanoporous structure found in rice husks is more suitable for solving the volume expansion problem in LIBs than any other artificially developed nanostructure. In particular, most of the artificially developed nanostructures have an issue of irregular particle sizes, but the structure from rice husks holds very uniform particles and pores.
As the silicon technology in the battery industry gets more mature, it could be possible that one of your next smartphones contains a part that grew on a rice field.