Rice is one of the top three staple foods in the world. But in the future, it may be a key part of the battery in your plug-in electric vehicle. Through a project supported by the Energy Department's Vehicle Technologies Office, researchers at Stanford University have been able to produce silicon structures for lithium-ion batteries from rice husks, a waste product of this ubiquitous agricultural crop.
Silicon anodes for lithium-ion batteries can store 10 times more capacity than graphite anodes, the material most commonly used in today's batteries. However, the cycle life of batteries that use silicon anodes - how many times a battery can charge and discharge while maintaining its capacity - is much shorter than those that use graphite. Another major challenge is that silicon expands by as much as four times its initial volume when the battery is charged.
One potential solution may be designing anodes that use silicon nanostructures, structures that are larger than molecules, but too small to be seen with a typical microscope. Although researchers have developed silicon nanostructures that enable longer cycle lives, they are expensive. They also have low mass loading, where there is a small amount of mass of silicon per area of battery electrode, which reduces their capacity to store energy.
However, synthesizing silicon nanostructures from rice husks may address some of these challenges: they're extremely abundant, and at $18/ton, are quite cheap. Like the seeds in a pomegranate, the nanoparticles maintain contact with each other during battery cycling while still having space to expand. This combination of traits allows the anode to maintain its energy storage capacity without sacrificing longer cycle life.
The Stanford researchers went through several steps to take the rice husks from the field to the laboratory:
They first burned them in air to convert them to pure silica. After exposing the silica to magnesium, the rice husks produced a silicon structure made of physically interconnected nanoparticles. The small size and porous nature of the new nanostructure gives it superior performance as a battery anode over both commercial particles.
After producing the nanostructures, the researchers tested them in battery cells. So far, the results are very promising. Because of their small size, high porosity, and interconnectedness, the developed nanostructures exhibited higher capacity, cycle stability and cycle life.
To follow up on the progress made, the researchers are investigating how to develop a scalable and low-cost method for creating the silicon nanostructures that also maintains their performance. They are also doing research to better understand how particles affect each other, develop larger versions, and test them further.
While silicon nanostructures from rice husks are still a long ways from being produced on a mass scale, one day they may be as common on the road as they are on the plate.
Funded by the Energy Department's Vehicle Technologies Office, this project was part of the Batteries for Advanced Transportation Technologies (BATT) program—now incorporated in the Advanced Battery Materials Research (BMR).
Source and top image: US Department of Energy
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