Scrapped car tires have been used for construction material and recycled into floor mats and shoe soles. Now they might find their way back into hybrid and electric cars, but under the hood. Scientists at Oak Ridge National Laboratory have devised a method to convert tire rubber into a carbon material for lithium-ion battery anodes.
Conventional lithium anodes are made of the graphite, a natural carbon material that is mined. The Oak Ridge researchers, led by materials chemist Parans Paranthaman, have found an alternative waste source for the carbon anode.
In a paper published online in the journal RSC Advances, they outlined a straightforward process to convert waste tire rubber into nanoporous carbon. They first pulverize waste tire rubber into tiny micrometer-sized pieces, which they break down in a hot sulfuric acid bath. The resulting rubber slurry is filtered, washed and turned into a solid cake. Heating the cake in the presence of nitrogen gives a highly porous carbon black material with pores that were less than 2nm in diameter.
The researchers made a small, lab-scale battery using the carbon material as the anode. The anode’s capacity — a measure of the charge it can store per gram of weight — was 390 milliamp-hours per gram (mAh/g). That’s slightly better than graphite’s 370 mAh/g capacity.
The tire rubber-derived carbon maintained that capacity after 100 charge-discharge cycles. But the recycled anodes will need to prove their mettle for the thousands of charge cycles that real-world batteries undergo.
Plus, they will have to compete with several other high-capacity anode materials that researchers are tinkering with in attempts to make longer range, longer-lasting Li-ion batteries. There is, for instance, a big research push to replace graphite anodes with nanostructured silicon, which take advantage of silicon’s 4000 mAh/g Li-storage capacity. Graphene is also a strong contender for Li-ion anodes. Meanwhile, some efforts are focused on making both the anode and cathode with carbon nanomaterials, in one case derived from cotton fibers.
For now, though, graphite is the anode material-of-choice. And the new technique offers an inexpensive, environmentally friendly way to make an anode material that’s better than graphite. “Using waste tires for products such as energy storage is very attractive not only from the carbon materials recovery perspective but also for controlling environmental hazards caused by waste tire stock piles,” Paranthaman said in a news release.