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.
“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” said Ville Piippo, CTO at Donut Lab. “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.”
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.
Intertek also performed a nail penetration test on Donut Lab's next-generation battery cell to evaluate the cell's behaviour 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 worse-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 production of bipolar cells is in itself as big a leap as the transition from standard lithium-ion batteries to solid-state ones” added Marko Lehtimäki, CEO of Donut Lab. “Bipolar cells open up possibilities that were previously unfeasible - they maximise the energy density of batter packs and minimise their structural complexity and cost.”
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