Rechargeable lithium-ion batteries are important in our lives as we use them frequently in our smartphones, laptops, tablets, power tools, and electric vehicles. They are even used to help power the Mars Curiosity rover! Typically, graphite, the form of carbon found in pencils, is commonly used as an active anode material in lithium-ion batteries. In a battery, electrons flow from the anode (the negatively charged component) to the cathode (the positively charged part). Natural graphite can be mined, which is expensive and environmentally destructive, or artificial graphite can be produced, which is time-consuming and expensive. As lithium ion batteries are in high demand from consumers around the world, the demand of graphite continues to rise.
In this study, researchers realized that soot expelled from diesel engines on merchant ships is an abundant source of carbon that could be recycled for use in lithium-ion batteries. While it is forbidden by MARPOL Annex V (the International Convention for the Prevention of Pollution from Ships) for ships to discharge soot into the sea, marine diesel engines can annually emit millions of tons of exhaust particles. As there is not yet a clear way to clean soot from ships at sea, the ships must be cleaned on land. Over 1,000 liters of soot per year are collected from one Panamax (mid-size ocean-going container) ship’s economizer, which is an engine-associated area where the largest amount of soot accumulates. For this study, the researchers obtained soot from economizers on ships currently in operation.
In order to make graphite, you need to find a suitable carbon source and heat it to very high temperatures (through a process called pyrolysis) to eliminate any organic matter, which leaves elemental carbon. In this study, the ship’s engine performed these steps, which left only the graphitization (transformation of the carbon to a graphite-like structure) process to be performed by the researchers. The graphitization involved heating the soot incrementally to 2,700°C in an ultra-high temperature furnace under an argon gas atmosphere.
After this, the soot was cooled to room temperature and analyzed using high powered microscopes and a tool called x-ray diffraction. The microscope revealed that the soot was transformed from its original form and now resembled an almost perfectly crystalline graphite structure. The X-ray diffraction technique showed that the original soot contained many impurities such as sulfur, nitrogen, or hydrogen before heating. After this process, these impurities were gone. Their absence may have helped to form the more crystalline carbon structures observed.
The researchers then tested the newly formed product for its electrical conductivity. They added it to an anode slurry composed of carbon black (a conductor of electricity) and a binding agent, which was then dried on copper foil to be used in coin-shaped batteries with lithium coin chips as counter and reference electrodes. The cycling performance of this soot-based anode compared to the commonly-used artificial graphite as the anode active material were found to be very similar. This showed that recycled soot is indeed a promising source as an anode active material in rechargeable lithium-ion batteries.
While this is a promising renewable technology, only one source of soot was used for this pilot study. The authors note more research will have to be done on soot from other ships and engine sources to further test this renewable alternative.