By ATN Editorial Team
Summary: VTT has measured the Donut Battery energy density at 409 watt-hours per kilogram and 805 watt-hours per litre, and that measurement is the new result in this announcement. Donut Lab restates alongside it the Intertek findings it first published on 28 August 2026, which confirmed a bipolar structure inside its next-generation cell and recorded no thermal runaway when a nail was driven through a fully charged cell.
Key engineering takeaway: The Donut Battery energy density figure is a cell-level number from a single energy-optimised cell, charged and discharged at 25 degrees Celsius across the full voltage range, so the pack-level figure will be lower once housing, cooling, busbars and cell-to-pack overhead are counted. It is also the one claim in the series that cannot yet be checked. Donut Lab publishes the underlying reports at idonutbelieve.com, where the VTT fast-charge, high-temperature, self-discharge, safety and swelling reports are all downloadable in full, but no energy density report had been posted there at the time of writing.
Why it matters: The architectural finding is more consequential for pack engineering than the Donut Battery energy density headline. Intertek reports multiple electrochemical layers connected in series inside one can, with split metal foils acting as electrodes on both sides of each layer, so cell voltage is raised internally rather than by wiring separate cells in series externally. That removes intercell busbars, terminals and much of the module structure, which is where a good deal of pack mass and cost currently sits. Two caveats travel with it. Intertek states that issuing its reports does not constitute certification, endorsement, approval or guarantee of the product or the manufacturer, and the nail penetration result is a single abuse-tolerance data point rather than a safety certification.
Technology company Donut Lab has today published new test results. According to the report by the research organisation VTT, the Donut Battery achieved an energy density of 409 watt-hours per kilogram and 805 watt-hours per litre.
“This was a fairly straightforward test, as energy density consists of two things: how much energy a battery cell holds and how much it weighs or takes up space. The test used a battery cell with the same chemistry as in VTT’s previous tests, only energy optimised as part of our broader development work,” says Ville Piippo, CTO at Donut Lab.
How VTT Measured The Donut Battery Energy Density
At the start of the test, the cell was weighed and measured. The battery was then fully charged and discharged at 25 degrees Celsius using the entire voltage range of the cell – this was used to determine how many watt-hours could be extracted from the cell. Dividing energy by mass gives the gravimetric energy density of the battery. Dividing energy by volume gives the volumetric energy density.
Tailored Cells Rather Than One Standard Battery
“Today’s test result and the other features that were demonstrated this spring not only show that the Donut Battery is an exceptional innovation. They also reflect the limitless possibilities of our technology, which we can increase with relatively light optimisations. After the launch, we have focused on manufacturing the next generation of cells, and we plan to scale their production to an industrial level,” says Marko Lehtimäki, CEO of Donut Lab.
Donut Lab’s customer base is diverse and ranges from vehicles to drones and battery energy storage systems.
“Our key goal is to develop battery solutions that meet the technical needs of each customer. Instead of a single standardised battery, we develop tailored variations of the same material and continuously optimise the technology so that it is suitable for the requirements of different industries. To support our development work, we have also wanted to identify several different actors for testing and verifying our technology,” Lehtimäki continues.
Intertek Confirmed A Bipolar Cell Structure
Donut Lab has also launched third-party research into the next generation of its battery technology cells, in collaboration with the international testing and certification company Intertek. The company examined the cell’s internal architecture, which confirmed the cell’s bipolar structure.
In its structural examinations, Intertek found multiple electrochemical layers connected in series inside a single cell. Split metal foils act as electrodes on both sides of the layers. This is a bipolar cell structure, where a higher voltage is generated inside the cell by layers connected in series, rather than separate cells being connected in series externally.
“A bipolar cell structure is possible only with a solid electrolyte. Any liquid would seep between the layers and allow ions to flow to the wrong places. This is why bipolar cells are not made with liquid electrolytes. This ultimately proves that our batteries are solid-state,” Piippo explains.
“The production of bipolar cells is in itself as big a leap as the transition from standard lithium-ion batteries to solid-state ones. Bipolar cells open up possibilities that were previously unfeasible – they maximise the energy density of battery packs and minimise their structural complexity and cost,” Lehtimäki adds.
Nail Penetration Test Left The Cell Stable
Intertek also performed a nail penetration test on Donut Lab’s next-generation battery cell to evaluate the cell’s behavior in a severe mechanical damage scenario. In the test, a steel nail was driven through a fully charged battery cell and left in the cell for an hour to simulate a permanent internal short circuit.
In a typical lithium-ion battery with a liquid electrolyte, this is a worst-case scenario. The short circuit discharges energy to a single point, the temperature rises, the liquid electrolyte is flammable, and the cell can undergo thermal runaway, causing smoke, fire and even an explosion. In the test conducted by Intertek, the Donut Lab battery cell remained completely stable throughout the hour-long monitoring.
“The nail penetration test proves in its simplicity that our battery is very safe. Safe cells also make construction simpler, lighter and more cost-effective at the battery pack level,” Piippo summarises.
VTT’s and Intertek’s test reports are available at Idonutbelieve.com.
Frequently Asked Questions
What is the energy density of the Donut Battery?
VTT measured 409 watt-hours per kilogram gravimetric and 805 watt-hours per litre volumetric. Donut Lab says the cell used the same chemistry as VTT’s earlier tests but was energy optimised. Both figures are cell-level, not pack-level. Donut Lab publishes its VTT and Intertek reports at idonutbelieve.com, although the energy density report had not been posted there when this was written.
What is a bipolar battery cell?
A bipolar cell stacks several electrochemical layers in series inside a single housing. Instead of each layer having its own separate positive and negative current collectors that are then wired together outside the cell, a shared metal foil acts as the positive electrode for one layer and the negative electrode for the next. The cell therefore delivers a higher voltage on its own terminals without external series connections.
Why does a bipolar cell need a solid electrolyte?
Because the layers sit directly on top of one another and share foils, any liquid electrolyte would migrate between layers and provide an unintended ion path, shorting the stack internally. Donut Lab CTO Ville Piippo makes this argument the other way round: because Intertek found a working bipolar structure, the electrolyte must be solid.
What does a nail penetration test show?
It forces a permanent internal short circuit by driving a steel nail through a fully charged cell. In a conventional liquid-electrolyte lithium-ion cell the energy discharges into a single point, heat builds and the flammable electrolyte can drive thermal runaway. Intertek reports that the Donut Lab cell stayed stable for the hour it was monitored. Intertek notes that issuing its reports does not constitute certification, endorsement, approval or guarantee of the product or the manufacturer, so this is a single abuse-tolerance data point rather than a safety certification.
How does 409 Wh/kg compare with current lithium-ion cells?
High-nickel NMC cells in production electric vehicles typically sit in the region of 250 to 300 watt-hours per kilogram at cell level, and LFP cells lower again. A verified 409 Wh/kg cell would therefore be a substantial step, though the comparison that matters for vehicle engineers is at pack level, where cell-to-pack efficiency, thermal management and structural mass decide how much of the cell figure survives.
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