When General Motors briefly first wowed the world with its EV-1 electric car, back in 1990, it relied on lead-acid batteries that packed a piddling 30 to 40 watt-hours per kilogram. The project eventually died, in part because that metric was so low (and the cost was so high).
It was the advent of new battery designs, above all the lithium-ion variant, that launched today’s electric-car wave. Today’s Tesla Model 3’s lithium-ion battery pack has an estimated 168 Wh/kg. And important as this energy-per-weight ratio is for electric cars, it’s more important still for electric aircraft.
Now comes Oxis Energy, of Abingdon, UK, with a battery based on lithium-sulfur chemistry that it says can greatly increase the ratio, and do so in a product that’s safe enough for use even in an electric airplane. Specifically, a plane built by Bye Aerospace, in Englewood, Colo., whose founder, George Bye, described the project in this 2017 article for IEEE Spectrum.
The two companies said in a statement that they were beginning a one-year joint project to demonstrate feasibility. They said the Oxis battery would provide “in excess” of 500 Wh/kg, a number which appears to apply to the individual cells, rather than the battery pack, with all its packaging, power electronics, and other paraphernalia. That per-cell figure may be compared directly to the “record-breaking energy density of 260 watt-hours per kilogram” that Bye cited for the batteries his planes were using in 2017.
Oxis Energy developed this lithium-sulfur battery cell and will now test its feasibility for use in an electric airplane. Photo: Oxis Energy
This per-cell reduction will cut the total system weight in half, enough to extend flying range by 50 to 100 percent, at least in the small planes Bye Aerospace has specialized in so far. If lithium-sulfur wins the day, bigger planes may well follow.
“We believe this to be the first phase in the electrification of commercial aircraft and will ultimately form the basis for the electrification of air taxis, with the additional requirement for regional aircraft,” said Huw Hampson-Jones, the chief executive of Oxis, in a statement.
One reason why lithium-sulfur batteries have been on the sidelines for so long is their short life, due to degradation of the cathode during the charge-discharge cycle. Oxis expects its batteries will be able to last for 500 such cycles within the next two years. That’s about par for the course for today’s lithium-ion batteries.
Another reason is safety: Lithium-sulfur batteries have been prone to overheating. Oxis says its design incorporates a ceramic lithium sulfide as a “passivation layer,” which blocks the flow of electricity—both to prevent sudden discharge and the more insidious leakage that can cause a lithium-ion battery to slowly lose capacity even while just sitting on a shelf. Oxis also uses a non-flammable electrolyte.
Presumably there is more to Oxis’s secret sauce than these two elements: The company says it has 186 patents, with 87 more pending.
This story was updated on 14 November 2019.
Philip E. Ross is a senior editor at IEEE Spectrum. His interests include transportation, energy storage, AI, and the economic aspects of technology. He has a master's degree in international affairs from Columbia University and another, in journalism, from the University of Michigan.