Burned Rice Hulls Could Help Batteries Double Their Energy Capacity

Researchers from the University of Michigan determined that burned rice hulls could help batteries store more charges. A close inspection of ash from burned rice hulls, which are the hard outer layer of grains, revealed a form of carbon that could nearly double the energy density of typical lithium-ion or sodium-ion batteries. The carbon dramatically increases batteries’ energy density, allowing them to last up to twice as long.

The researchers made this discovery by producing hard carbon through the combustion of rice hull ash. This method is more sustainable and efficient than traditional methods that require high temperatures and an oxygen-free environment. Additionally, it could serve as a domestic alternative to imported graphite which can reduce the reliance on foreign sources for making battery electrodes.

Rice hull ash is primarily composed of silica (about 90%) and carbon (about 10%). The research team developed a method to extract high-purity silica form the rice hull ash. The researchers noted, “Once the silica is partially removed from the rice hull ash through a process called depolymerization, the remaining ash is about 60%-70% carbon.”

Initially, the carbon residue from rice hull ash was believed to lack a defined structure. However, advanced spectroscopic analysis revealed the presence of tiny graphite structures embedded within the amorphous carbon matrix. Known as hard carbon, this unique combination exhibits enhanced properties compared to pure amorphous carbon.

Richard Laine, corresponding author of the research, stated, “Hard carbon can be produced by combustion in this case because as you burn away the carbon of rice hulls, you create a shell of silica around the remaining carbon and it bakes it like a pie.”

During testing, the hard carbon derived from rice hull ash demonstrated superior electrochemical performance compared to commercial hard carbon and graphite, suggesting that it could be used as the anode in lithium-ion batteries.

According to the research published in Advanced Sustainable Systems, hard carbon batteries can store significantly more energy than traditional graphite batteries. A gram of graphite can store around 370 milliampre-hours (mAh) of electrical energy. On the other hand, a commercial hard carbon can store about 500 mAh per gram.

The rice hull ash hard carbon surpasses both commercial hard carbon and graphite. The nanoporous structure enhances the hard carbon’s lithium storage capacity. The research done by team from University of Michigan could address the rising demand for batteries in electric vehicles and renewable energy storage.