Heat That Helps, Not Hurts

Donut Lab has released a second set of independent test results for its controversial solid-state battery, and the findings challenge conventional assumptions about how batteries behave under extreme heat. Finland's VTT Technical Research Centre subjected a 26-amp-hour, 94-watt-hour cell to high-temperature discharge testing — and instead of degrading, the battery actually performed better.

At 80 degrees Celsius, the cell delivered 27.5 amp-hours, representing 110.5 percent of its room-temperature capacity. Pushed further to 100 degrees Celsius with a reduced discharge rate, it achieved 27.6 amp-hours — 107.1 percent of the baseline measurement. The cell was then charged normally, demonstrating that the extreme heat exposure had not damaged its fundamental electrochemistry.

For a conventional lithium-ion battery, this scenario would be catastrophic. Standard cells experience thermal runaway at temperatures well below 100 degrees Celsius, a process that can lead to fires and explosions. The Donut Lab cell's ability to not only survive but improve at these temperatures speaks to the fundamental difference between solid-state and liquid electrolyte architectures.

Why Solid-State Behaves Differently

The results align with theoretical expectations for solid-state electrolytes, where ionic conductivity — the ability of lithium ions to move through the material — improves with increasing temperature. In a conventional battery, the liquid electrolyte begins decomposing at high temperatures, generating gases and creating dangerous conditions. A solid electrolyte avoids this failure mode entirely.

However, VTT's testing did reveal one concerning observation: after the 100-degree discharge, the cell pouch had lost its vacuum seal. While the battery continued to function, the vacuum loss suggests gas generation at extreme temperatures — a finding that raises questions about long-term durability under repeated high-temperature exposure.

This nuance is precisely what independent testing is designed to surface. Donut Lab's own testing would report the headline performance numbers; VTT's independent evaluation adds the kind of detailed observations that inform realistic assessments of the technology's readiness.