Last year, lithium-ion battery fires became a hot topic, pardon the inescapable pun, with both a Tesla automobile and the Boeing 787 Dreamliner succumbing to fires. In cross-disciplinary research at the University of North Carolina (UNC), a compound being studied to prevent marine life from sticking to ships may also be the solvent (and the solution) to keep lithium ion batteries from catching alight when they overheat.
The surprising chemical in question is a lubricant called perfluoropolyether (PFPE), which is traditionally used to a heavy-duty lubricant for plasma etching equipment or aircraft fuel systems. Generally polymers don’t mix with electrolytes, yet not only does PFPE combine with lithium salts, but it retains its nonflammability and is thermally stable beyond 200º C (392º F). In contrast, the common solvent in commercial lithium batteries begins to degrade at 34º C (93º F) and has a low flashpoint. The team confirmed the stability of these compounds with every lab assistant’s dream experiment, setting things on fire.
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Furthermore, the PFPE-lithium combination exhibits characteristics pointing to it not just being a safe battery, but an excellent battery. Scientists commonly refer to the transference number and electrochemical polarization of batteries, both of which relate to battery life and performance.
In measuring the transference number, the team came to the surprising discovery that PFPE carries most of its current in the cation (the positive ion), resulting in a transference value higher than most other solvents. Though the conductivity of the PFPE mixture is relatively low, previous research suggests that high transference can make up for low conductivity.
Finally, all of this doesn’t matter if an electrolyte can’t operate well with a cathode and anode. The researchers charged and discharged the PFPE lithium battery with high-voltage cathodes and established that the compound did so stably and was promising for use in grid batteries that need to cycle within eight hours.
While the team acknowledges the necessity of further research into increasing the conductivity and cyclability of the battery, the finding is promising for large batteries used in aircraft and automobiles where overheating becomes an issue, and even in the reverse, in cold environments where conventional batteries fail.
Earlier this month we saw research that solved the flammable battery problem by using a wax as an electrolyte that melted when it became too hot, severing the connection with the cathode and anode.
The research team at UNC, led by Joseph DeSimone, first published their research in the Proceedings of the National Academy of Sciences.