If you’ll recall, during the extreme heat tests, the pouch surrounding Donut’s battery lost its vacuum seal. This led to researchers at Finland’s state-owned VTT Technical Research Centre, which conducted all of the lab’s previous tests, to explore whether the cell could still function under these high-stress, high-temperature conditions. As Donut notes, damage such as the one sustained by its pack during the heat test has been known to cause fire or thermal runaway in conventional lithium-ion batteries. The startup wanted to see whether its solid-state battery could avoid a similar calamity.
Researchers conducted three tests: a baseline test of 5 cycles at 1C (26 amps); a high-stress test of 50 cycles at 5C (130 amps) ; and another final baseline test of 5 cycles at 1C to measure degradation. After the tests, the team observed a sharp drop in energy capacity, from 24.7 Amp-hours to 11.2 Ah, or a loss of approximately 55 percent. They also recorded a drop in efficiency from 89.6 percent to 83 percent, and noted a 17 percent increase in the pack’s thickness.
Given the damage sustained by the pack, the results shouldn’t be that surprising. The loss of the vacuum seal certainly accelerated the battery’s degradation and contributed to its thick, swollen state after the test. Still, Donut Lab said this was further proof that its battery can avoid some of the worst outcomes of a damaged lithium-ion battery.
“No temperature spikes, no fire risk,” the company said on its IDonutBelieve.com site. “In this scenario the Donut Battery fails gracefully when damaged, continuing to operate safely at reduced capacity rather than posing danger to the user.”
But the tests also represent the closest that Donut Lab’s battery has come to a cycle test — and it was under these damaged conditions. The startup has yet to demonstrate how its pack holds up under an accelerate aging procedure that repeatedly charges and discharges the cells to evaluate their long-term performance and safety. Donut has previously claimed its solid-state battery can last 100,000 cycles, which would represent approximately 270 of charging and discharging. This would be orders of magnitude greater than the 1,000-2,000 full charging and discharging cycles that the average EV battery can withstand before its capacity starts to degrade.
And more importantly, Donut has yet to ask for an independent verification for its claimed 400 Watt-hours-per-kilogram of energy density. That seems like a simple enough test — all you have to do is weigh the cell and measure its output — and it’s unclear why Donut Lab hasn’t asked for it yet.
We’ll have more to say about Donut Lab’s tests and what independent experts are saying about the startup in the weeks to come.