The search for a sustainable EV battery has taken the iconic automaker Mercedes-Benz in some strange directions. The latest news involves a new method for extracting hard carbon from rice hulls, leading to new EV battery anodes that outperform conventional -- and less sustainable -- anodes made from conventional graphite.
The Sustainable EV Battery Of The Future, From Lignin
Rice hulls are the latest twist in the race to decarbonize the EV battery supply chain. Conventional lithium-ion batteries rely on graphite, which can be mined from the earth or synthesized from fossil fuels. Either way, much of the US graphite supply comes from overseas. Both versions also involve carbon emissions among other environmental impacts, which presents a challenges for automakers seeking to claim the #1 sustainable brand for themselves.
Last year researchers at Northwestern University released the results of a first-of-its-kind, soup-to-nuts analysis of graphite usage in the US and noted a good deal of waste, much of it involving refractories and foundries with a lesser amount from EV battery makers. "Recycling more graphite and producing graphite from lignin would favorably influence today's supply chain," the research team concluded.
From a sustainability perspective the lignin pathway is particularly promising because it can help divert agricultural waste from landfills and into a more useful second life, replacing fossil-sourced synthetic graphite.
From a science perspective, though, the biomass pathway is challenging. The conventional method is to extract carbon from burned-up biomass, but that yields a disordered from of carbon, not graphite. Exposing biomass to extremely high heat yields a higher-quality carbon product, but at the price of increased carbon emissions.
The Rice Hull Solution For The Sustainable EV Battery Of The Future
The new rice hull research project was conducted by a research team at the University of Michigan, with primary funding from the National Science Foundation along with the Research & Development North America branch of Mercedes-Benz.
The University of Michigan points out that producing EV battery-grade graphite from biomass generally involves five to 10 tons of carbon dioxide emissions per ton of graphite, based on a high-heat scenario.
That doesn't mean the biomass-to-graphite pathway is a lost cause, sustainably speaking. Instead, the researchers advocate for selecting specific types of biomass that are more amenable to graphite production. They zeroed in on rice hulls partly due to the availability of a domestic supply chain for EV battery makers, with US producers generating about 20 billion pounds of rice annually.
The team also drew on on their previous research, which involved creating a method for extracting silica from rice hull ash. The silica can be repurposed as silicon for solar cells and other products, while the remaining ash is 60-70% carbon.
"The leftover carbon was thought to be shapeless and disorganized, a material called amorphous carbon," the school recounts. Upon further analysis deploying specialized spectroscopy equipment, the researchers determined that nanoscale "islands" of graphite were disbursed within the matrix. Instead of an amorphous mess, they found an ordered form of carbon called hard carbon.
As for how that came to be, it has something to do with the unique characteristics of rice hulls. "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," explains U-M professor Richard Laine, the corresponding author of the study.
Rice Hulls Outperform Graphite In Lithium-Ion Batteries
The big question is whether or not hard carbon made from rice hulls can match the performance of conventional graphite when deployed in the anode of an EV battery. The researchers arrived at the answer no, rice hulls don't match the performance. They exceed it.
"When testing the electrochemical properties of hard carbon obtained from rice hull ash, it outperformed both commercial hard carbon and graphite as the anode of a lithium-ion battery," U-M explains.
As described by the school, one gram of conventional graphite in an EV battery can accept about 370 milliampere-hours (mAh). Conventional hard carbon does much better, at about 500 mAh, but the hard carbon produced from rice hull ash outperformed both by a wide margin, at 700 mAh.