Donut Lab Survives 100º C Test. Is That Good Enough?

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The furor over the claims made by Donut Lab for its “solid state/not so solid state” battery continues unabated. This week, it released new testing data for its breakthrough battery that purports to show it worked admirably at temperatures of 80º C and 100º C — performing better at higher temperatures than it does at room temperature — and yet the controversy continues.

In an effort to combat the negative publicity, the company has created the I Donut Believe website, where it assembles all the test results that support its claims. To date, they all originate from VTT, the Finnish state research institute, which has been asked to counter the skeptics by providing independent validation of the company’s claims.

The website introduces itself this way: “This website serves as the platform for publishing validation results and technical documentation related to the Donut Solid State Battery. Donut Lab has commissioned the internationally renowned VTT Technical Research Centre of Finland to conduct independent measurements of the battery’s performance and key characteristics.

“Unless otherwise stated, all tests presented here have been conducted by VTT. Full reports will be published alongside detailed video documentation that walks through the procedures, showcases the test setups, and explains the results.”

My colleagues Tina Casey and Christopher Arcus have written several articles lately that support the company’s claims. According to the company, its battery has an energy density of 400 watt-hours — nearly double that of typical lithium-ion cells, a five-minute charge time, an operating range of -30º C to 100º C, a 100,000 cycle lifespan, and zero rare earth materials.

This week, Donut Lab released new data from VTT from testing at 80º C and 100º C. Are those results enough to quell the doubts about this new battery technology? Not even close. As InsideEVs reports, “Once again, the data is raising as many questions as it answers.”

The Results Of The Second Testing Protocol

Electrive says VTT tested a Donut Lab cell with a nominal capacity of 26 Ah and a nominal energy content of 94 Wh according to the datasheet, in a Weiss LabEvent T/110/40/3 climate chamber using a PEC ACT0550 cell tester. Unlike the first test series, the results of which were published last week, VTT placed a 2.4-kilogram steel weight on the cell this time to apply mechanical pressure. The temperature sensor, critical for the high-temperature tests, was positioned underneath the cell between the cell and the aluminium heat sink used in the first test.

The test series comprised three charge-discharge cycles, beginning at 20º C in the climate chamber. After a one-hour stabilization period, VTT charged the cell at a constant current of 24 amperes until it reached 4.15 volts. Charging then continued at 4.15 volts until the current dropped to 1.2 amperes, or 0.05 C. For the first test, VTT discharged the cell after one hour at a constant 14 amperes until the voltage fell to 2.7 volts. This procedure determined a capacity of 24.9 Ah for the cell.

After fully recharging the cell using the standard procedure outlined above, VTT increased the temperature in the climate chamber to 80º Celsius and maintained it at that level for two hours. At that temperature, VTT discharged the cell at 24 amperes down to 2.7 volts to determine its capacity under elevated temperature conditions.

Following a one-hour stabilization period at 20º C, VTT recharged the cell and held it at full charge for one hour. It then conducted a reference discharge at 20º with a current of 12 amperes down to a cell voltage of 2.7 volts.

For the third test, VTT heated a fully charged cell under the same experimental conditions to 100º C and kept it at that temperature for two hours. It subsequently discharged the cell at 12 amperes down to 2.7 volts. The institute then reduced the temperature back to 20 degrees Celsius and, after one hour, recharged the cell completely. Detailed measurement data are available in the VTT report.

“Under the specified conditions, the cell was successfully discharged at +80° C with a current of 24 A, achieving a discharge capacity corresponding to 110.5 percent of the initial discharge capacity at +20° C with the same current. After discharge, the cell could be charged normally, and no observable changes were detected,” VTT wrote. “The cell was also discharged at +100° C with a current of 12 A, achieving 107.1 percent of the reference discharge capacity measured at +20° C with the same current. After discharge, the cell could be charged normally.”

Finding The Silver Lining

In a statement, Donut Lab said, “The test measuring battery performance at high temperatures has shown that the Donut battery is extremely heat resistant and even delivers improved performance at elevated temperatures.”

“The full capacity of the battery was utilized at both 80º and 100º C with excellent results. Discharge at a 1C rate at 80 degrees and at a 0.5C rate at 100 degrees was possible without any temperature increase,” added Ville Piippo, CTO of Donut Lab. “The battery characteristics remained unchanged even at 100 degrees, and after recharging it functioned flawlessly, confirming the convincing performance of the Donut battery even under extremely hot conditions. This demonstrates that the battery is exceptional not only in terms of performance but also in safety.”

Sounds pretty good, doesn’t it? Yes, it does, except for a few things. Donut Lab says it has created a solid-state battery, but VTT through a curveball into the testing report, saying only that it conducted tests on an “energy storage device provided by the customer […] identified by the customer as a solid-state battery cell.” Hardly a ringing endorsement of the company’s claims.

In addition, VTT reported that during the second high-temperature test, the pouch cell labelled DL2 lost its vacuum after the 100º C test. However, what would normally be considered damage Donut Lab said was proof of success. “The battery and its active materials remained fully functional even after the outer cell casing lost its vacuum at 100 degrees.”

“Conventional lithium-ion batteries are highly sensitive to heat and their maximum operating temperature typically lies around 60 to 70 degrees Celsius. At high temperatures, the risk of cell damage increases, reducing service life,” said CTO Piippo. “In addition, high temperatures can accelerate reactions inside the cell and, in the worst case, significantly increase the risk of thermal runaway. The Donut battery starts from a fundamentally different position, as it contains no flammable liquid electrolytes, and therefore the maximum operating temperature is higher.”

Not Breaking Any Laws

Assuming all that is true, Eric Wachsman, a professor at the University of Maryland’s Department of Materials Science and Engineering who specializes in solid-state batteries and solid-oxide fuel cells, said the insufficient data does not represent real-world usage in automotive applications. “The cells are not violating any law of thermodynamics,” Wachsman told Suvrat Kothari of InsideEVs. “But the data presented leaves a lot to be desired for many reasons.”

Wachsman said the pouch issue was a potential red flag. The loss of vacuum may indicate that the cell lost its “hermetic seal,” the barrier that keeps outside air away from the sensitive internal chemistry. “Pouch cells will tend to expand due to internal pressure as they can give off gas during cycling. It’s clear they experienced excessive swell after a few cycles,” he said after comparing the fast-charging images from last week’s test to this week’s high-temperature results.

Whether this is a major safety issue or something that’s expected on solid-state cells remains unclear. However, the broader consensus is consistent among experts. Neither test reveals meaningful pack-level performance over the thousands of cycles that actually matter for real-world use. “To be commercially relevant, the cells need to be stable with less than 10 to 20 percent of capacity fade for thousands of cycles,” Wachsman said. “Without that, the tests are essentially meaningless.”

And so the beat goes on. Is the Donut Lab battery solid state? Is it ready for prime time? Is it a genuine breakthrough or just a sideshow? We take no position on any of this controversy, but our readers, who are all well above average, may wish to express their opinions in the comments.