Finnish battery developer Donut Lab has released a second series of independent performance tests on its contested solid-state cell, focusing on high-temperature operation. The tests were conducted by VTT Technical Research Centre of Finland as part of Donut Lab’s “I Donut Believe” campaign, which aims to provide staged, third-party validation of the technology after its January unveiling at CES.
In the high-temperature assessment, VTT evaluated a 26 Ah, 94 Wh nominal cell in a controlled climate chamber equipped with a cell tester and an aluminium heat sink. This time, a 2.4-kilogram steel weight applied mechanical pressure, and a temperature sensor was placed between the cell and the heat sink. The series included three charge-discharge cycles.
At 20 °C, the cell was charged at 24 A to 4.15 V, then held at constant voltage until the current fell to 1.2 A. A 14 A discharge identified a capacity of 24.9 Ah. After a full recharge, the chamber temperature was raised to 80 °C and held for two hours. A 24 A discharge to 2.7 V yielded a capacity equal to 110.5% of the reference at 20 °C. Following another recharge and stabilization at 20 °C, the cell was heated to 100 °C for two hours. A 12 A discharge produced 107.1% of the reference capacity.
VTT reported that the cell recharged normally after each discharge, with no immediate abnormalities at elevated temperatures. However, the pouch lost its vacuum following the 100 °C test—a point Donut Lab characterizes as noncritical since the active materials remained functional.
Donut Lab’s CTO stated that the results demonstrate exceptional heat resistance and performance stability “even under extremely hot conditions,” attributing the robustness to the absence of flammable liquid electrolytes. VTT’s report, however, does not confirm whether the tested device is lithium-free, noting only that it was “identified by the customer as a solid-state battery cell.”
Critics remain skeptical. Some experts have questioned the cell chemistry and test protocols, while others have drastically lowered their confidence in the company’s claims. Industry observers agree that further independent analyses of the cell’s internal chemistry will be necessary to resolve ongoing debate.
Source: Donut Lab, VTT Report VTT-CR-00124-26
