Earlier this year, a relatively unknown startup from Finland made a startling announcement: It had finally solved solid-state batteries.

Not only that, but Donut Lab, a spinoff of Verge Motorcycles, said that its solid-state battery — long considered the “Holy Grail of batteries” for their high-density, durable, fast-charging abilities — would go into production later this year.

Battery experts were understandably skeptical. After all, solid-state batteries are one of those technologies, along with artificial general intelligence and the hyperloop, that seem perpetually two years away. And while most legitimate efforts in this field — whether academic or commercial — have some level of published research or recognizable names attached, Donut Lab seemed to have emerged out of nowhere, with no known researchers or prior presence in the field. This lack of traceability immediately raised concerns about the startup’s credibility.

“I can’t say they didn’t do it,” said Eric Wachsman, the director of the Maryland Energy Innovation Institute and an expert on solid-state batteries and solid oxide fuel cells. “All I can say is they haven’t demonstrated that they have.”

The skepticism seems warranted, especially when you consider how many other people have been chasing the solid-state dream. Were we really to believe this obscure startup had beaten Toyota, Stellantis, and the entire nation of China to the punch? The odds were against it.

Donut Lab seemed to anticipate the doubt, launching a website last February called idonutbelieve.com that would serve as a platform to publish independent tests verifying that, in fact, its solid-state battery was real, and spectacular. Over the course of several weeks, the startup posted third-party results from state-owned VTT Technical Research Centre of Finland that it said proved its battery was what it said it was: a fast-charging, high-energy-density solid-state battery that wasn’t actually a supercapacitor in disguise.

“The resistance won’t disappear when we present the proof,” Donut Lab CEO and cofounder Marko Lehtimäki said in a video. “It will just intensify because this new technology is a threat to the established players in the industry.”

But Donut Lab is still hiding the ball on some key information. At CES in January, the startup said its solid-state battery has an energy density of 400Wh per kilogram—roughly twice that of typical lithium iron phosphate (LFP) batteries in production. Not only that, but it could charge to full in five minutes, had a practically unlimited lifespan of 100,000 charging cycles, was unaffected by heat and cold (negative 30 degrees Celsius and 100C), and contains no rare earth elements, precious metals, or flammable liquid electrolytes.

Much of that remains unsubstantiated. Even after posting five independent test reports from VTT, the startup has yet to demonstrate three of the most important metrics: chemistry, density, and cycle-life claims.

The stakes are incredibly high. Imagine an electric vehicle that can travel 700–800 miles on a single charge, and that wasn’t at risk of bursting into flames because the flammable electrolytes had been replaced with a solid material.

In lithium-ion batteries, the motion of the liquid electrolytes generates heat, and in certain situations, this can slip into a “thermal runaway” effect that results in a fire. By comparison, solid-state batteries would make it safer to quickly draw power from (or add it back to) the battery, meaning you could theoretically charge an EV faster. It also could mean, structurally, less room has to be devoted to temperature control, which could allow companies to squeeze more battery cells into the same size pack.

After reviewing the tests of the Donut battery, Wachsman said there are still significant concerns. During the extreme heat tests, for example, the pouch surrounding Donut’s battery lost its vacuum seal. Gas generation inside batteries — caused by processes like electrolyte decomposition or oxygen release — can lead to swelling and rupture of the battery pouch. But without knowing the exact chemistry of the cell, it’s difficult to say how significant it is that Donut’s battery had this failure.

Setting aside the Donut battery for a moment, solid-state batteries have struggled to graduate from the laboratory to the assembly line because of well-documented problems. These batteries are often plagued by the formation of metallic cracks called dendrites that cause them to short circuit. Think of them like cracks that form on a sidewalk when a tree root grows underneath.

Dendrites have been a thorn in the side of battery developers since the 1970s. One reason lithium-ion batteries have become ubiquitous while other approaches have stalled is that their commonly used graphite anodes are less susceptible to dendrite formation.

But new discoveries could help engineers finally overcome these hurdles. A research team from MIT recently published a study in Nature that found that chemical reactions caused by high electrical currents that weaken the electrolyte also make it more susceptible to dendrite growth. That’s why developing stronger electrolytes alone hasn’t solved the decades-old dendrite problem. And it could point to the importance of developing more chemically stable materials to finally fulfill the promise of solid-state batteries.

Progress is already being made — where else? — in China. Last month, CATL, which controls nearly 40 percent of the global battery market, filed a patent application for solid-state batteries with a reported 500Wh energy density. According to CarNewsChina, the battery maker has already been planning small-scale production in 2027. But automotive-grade cells won’t be ready likely until the end of the decade.

Other Chinese companies are rushing ahead. Automaker FAW said recently that its “liquid-solid-state” lithium-rich manganese cell with 500Wh/kg was ready for vehicle integration.

China is already laying the groundwork for mass production by the end of the decade, by which point it hopes the technology will be mature. And why wouldn’t it? This is a country that has taken EVs and battery development seriously for years, allowing it to corner the market on much of the world’s supply.

Different companies are taking different approaches. For example, Honda is committed to sulfur-based electrolytes despite emerging alternatives. Last October, Toyota announced “the world’s first practical use of all-solid-state batteries in BEVs” by 2027 or 2028. And Mercedes, using a prototype battery from startup Factorial, was able to get an electric EQS sedan a real-world range of 749 miles.

“The companies probably have a ways to go,” said Alevtina Smirnova, director of the NSF Industry-University Cooperative Research Center for Solid-State Electric Power Storage. “Because there is no comparison to what is happening now in China to what is happening here in the US.”

For its part, Donut Lab is unperturbed by the skepticism around its claims. On April 1st, Lehtimäki posted a new video addressing some of the controversy surrounding its solid-state batteries. He also revealed that Donut Lab had created a second, more production-ready version of its battery that would start shipping to customers later this year.

There was a crucial admission: The widely discussed “100,000 cycles” figure was a design target, he said, not an experimentally verified result. Actual testing has been conducted over shorter cycles, with projections extrapolated based on known variables such as charge rate, temperature, and usage conditions.

He then pivoted to a more near-term project: Donut Lab’s latest merch drop, including a “tin-foil”-covered bucket hat.