Donut Lab has released results from its second independent test of the Donut Battery, a solid-state cell designed for production vehicles. Conducted by Finland’s VTT Technical Research Centre, the evaluation measured capacity performance at 80 and 100 degrees Celsius. The battery delivered better-than-rated capacity at both temperatures and returned to normal function afterward.
Highlights
- The Donut Battery reached 110% of its rated capacity at 80°C and approximately 107% at 100°C, outperforming its room-temperature baseline in both cases.
- No degradation was observed after high-temperature cycling. The cell recharged to its original 4.15 volts with the same charge capacity as at room temperature.
- The solid-state design eliminates flammable liquid electrolytes, raising the maximum safe operating temperature well above the 60–70°C ceiling typical of conventional lithium-ion cells.
- VTT’s full test report is available for download on the I Donut Believe website.
Why Heat Tolerance Matters
Conventional lithium-ion batteries face serious risks at elevated temperatures. Heat accelerates internal reactions, shortens service life, and in worst-case scenarios increases the likelihood of thermal runaway. According to Donut Lab CTO Ville Piippo, the Donut Battery starts from a fundamentally different baseline. It contains no flammable liquid electrolytes, which allows a higher maximum operating temperature.
How VTT Conducted the Test
The test setup was straightforward. Researchers placed a battery cell on an aluminum profile with a steel plate on top. The plate applied light pressure to hold the cell in place and reduce hot spots.
Testing followed a structured sequence:
- Room-temperature baseline: A standard 1C discharge test to a 2.7-volt cutoff established nominal capacity. The cell was then recharged to 4.15 volts.
- 80°C evaluation: The chamber temperature was raised to 80 degrees, and the cell stabilized for two hours. Researchers then repeated the 1C discharge test. After cooling, the cell was recharged and discharged again at 12 amps (0.5C) to define a secondary room-temperature baseline.
- 100°C evaluation: The cell was recharged, allowed to equilibrate for one hour, then heated to 100 degrees. After two hours of stabilization, the 0.5C discharge test was repeated.
- Post-test verification: The chamber returned to 20°C and the cell was charged back to 4.15 volts to confirm it retained full charge capacity.
Results Exceeded Expectations
At 80°C, the Donut Battery utilized up to 110% of its rated capacity. Internal resistance drops at higher temperatures, which reduces voltage loss during discharge and improves efficiency. The cell showed no visible changes after the test and continued to function normally.
At 100°C, the battery utilized approximately 107% of its rated capacity. It recharged to 4.15 volts with the same charge capacity as at room temperature. However, the cell’s external pouch lost its vacuum at that temperature. Despite this, the battery and its active materials remained fully functional.
Safety Implications for EV Applications
Piippo noted that the battery discharged at a 1C rate at 80 degrees and a 0.5C rate at 100 degrees without any temperature increase. Additionally, properties remained unchanged even at 100°C, and the cell functioned normally when recharged. He characterized the results as confirmation that the Donut Battery is exceptional in both performance and safety under extreme heat.
The results carry particular relevance for EV applications. Batteries in electric vehicles face sustained heat from fast charging, high ambient temperatures, and thermal loads from drivetrain operation. A cell that gains capacity rather than losing it under heat stress could reduce the need for aggressive thermal management systems.
About Donut Lab
Donut Lab is a technology company developing what it describes as the world’s first solid-state battery ready for production vehicles. The company publishes independent test results through its I Donut Believe verification site.
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