Donut Lab’s Solid-State Battery Charges Fast. But Experts Still Have Questions

• Donut Lab unveiled tests this week demonstrating its battery’s charging capability. 
• Experts still have big questions—about cycle life and performance at the pack level. 
• Donut Lab is releasing more evidence of its solid-state batteries, but there’s no word on the chemistry just yet.

It took a lot for Finnish startup Donut Lab to begin releasing evidence to support its extraordinary claims of having developed the world’s first production-ready all-solid-state battery for electric vehicles.

Its announcement at CES in January was met with intense skepticism, so now it’s releasing a series of independent tests to prove it’s legit. The company commissioned Finland’s state-owned VTT Technical Research Centre to independently test the charging performance of its solid-state cell. While the data looks impressive and supports at least one of the company’s claims, it also omits several details that battery experts consider critical.

Importantly, the test doesn’t clarify whether Donut’s cells are truly solid-state. “Anyone who does not reveal the chemistry cannot be the real deal,” Shirley Meng, a professor at The University of Chicago’s Pritzker School of Molecular Engineering, told me in an email.

Before digging into what the tests show and, just as importantly, what they do not, it’s worth revisiting Donut Lab’s claims. The startup, which has no proven track record in battery manufacturing, says it has beaten major automakers and cell manufacturers like Toyota, BYD, and CATL to the finish line of the solid-state race.

Donut Lab’s solid-state pouch cell with double-sided heat sinks.

Photo by: Donut Lab

Its solid-state battery, which will be installed in Verge Motorcycles’ EVs in the first quarter of this year, can apparently charge in five minutes, last 100,000 cycles and deliver 400 watt-hours per kilogram of energy density, all while remaining immune from thermal runaway. VTT’s test does show ultra-fast charging capability. It does not, however, address many of the other claims that are bundled into that bold promise.

VTT conducted a total of seven charging tests on a 94-watt-hour pouch cell. Four of those involved pumping electrons into the cell at rates of 5C and 11C. A rate of 1C means the battery can fully charge in one hour. 2C can do the same in 30 minutes. 5C is five times as fast as the one-hour rate. On the 5C test, the cell was charged at 130 amps and 4.3 volts.

It reached 80% in under 10 minutes and 100% in about 13 minutes, with a peak temperature reading of just 47 degrees Celsius (116.6 degrees Fahrenheit). The 11C charging test largely aligned with Donut Lab’s claims. It charged to 80% in just 4.6 minutes and was fully charged in under eight minutes, with a temperature reading of 63C (145F) only.

Photo by: Donut Lab

Both tests were conducted using single- and double-sided heat sinks to simulate real-world thermal management that allows excess heat to dissipate under high voltage and current loads. In practice, battery makers rely on far more sophisticated active cooling systems at the pack level. In that sense, simple heat sinks represent something close to a worst-case scenario.

So, the results show that Donut Lab’s cell can indeed charge extremely quickly—both to 80% and to 100%. If an electric car could recharge to 80% in under five minutes, that would revolutionize the industry and make refueling an EV about as quick as getting gas. It would vaporize one of the key pain points keeping people from going electric: fast-charging stops that typically take anywhere from 20-40 minutes.

Still, experts cautioned that lab-scale testing of a single cell says little about how an entire battery pack would behave in the real world.

“11C at lab scale is not unique,” Meng said. She has conducted research on solid-state batteries and energy storage materials and is of the opinion that lab results are limited in scope. “My lab has shown 20C in lab-scale cells, but it does not mean it can be translated to pack level in the real world.”

Photo by: Patrick George

Moreover, a handful of fast charging demonstrations in a lab does not demonstrate how a battery will behave over anything close to 100,000 cycles, which is what Donut Lab claims its design can endure. Charging super quickly with minimal degradation really only matters if that can be repeated over the lifetime of a vehicle. 

“The 11C charging rate is a promising indicator for all-solid-state batteries,” Jiayan Shi, an associate at research firm BloombergNEF specializing in electrochemistry, told InsideEVs. “However, the discharge rate and duration/capacity retention will be equally critical, as they directly influence how consumers can use the battery in real-world applications,” she added.

One interesting detail from the test was that the cell performed better when it was allowed to get hotter. When the researchers used less cooling (one heat sink instead of two), the cell temperature went up to 90 degrees Celsius (194 degrees Fahrenheit). VTT said this lowered its internal resistance and allowed it to charge faster. 

Traditional lithium-ion batteries, by contrast, typically struggle with high heat. As they get hotter, they often lose energy efficiency and can even become unstable or suffer permanent damage. Studies show that the sweet spot for the operating temperature for today’s lithium-ion batteries is between 25-40C (77-104F).

Photo by: Verge Motorcycles

Without confirmation of Donut’s underlying technology, whether the cell’s performance stems from a fundamentally new chemistry or clever optimization remains an open question. It’s worth noting that CATL’s most recent lithium-iron phosphate (LFP) batteries are capable of charging at peak rates of 12C, and BYD’s five-minute charging LFP battery that InsideEVs tested last year in China peaks at 10C. So even lithium-ion batteries are increasingly capable of ultra-fast charging speeds.

Donut Lab has faced criticism since it first unveiled the cells without disclosing meaningful technical details. The company has since acknowledged that negative media coverage made it harder to raise the capital needed to scale manufacturing. Its response now appears to be a recalibrated communications strategy: a slow, deliberate drip of information over time. That, and a defiant video doubling down on all of its claims.

For now, Donut Lab says it will release additional independent test results in the following weeks. When those arrive, the things to watch include battery chemistry, any information on energy density, and evidence of long-term durability. 

Until then, these results are intriguing, but far from definitive.

Contact the author: suvrat.kothari@insideevs.com