Donut’s Lab’s solid-state battery has hit 409Wh/kg: can this tech transform EVs?

The buzz continues around Donut Lab's solid state battery. So does the controversy. Today the Finnish/Estonian company announced test results by the Finnish state lab VTT for its cell's energy density: 409Wh per kilo. That's about half as much again as an average lithium-ion cell, or put another way two-thirds the battery mass for a given storage capacity.

And remember the earlier claim of 0-80 per cent charge in under five minutes. So if it works according to the claims, you'll have an electric car battery that weighs far less than today's for a given range, and charges faster.

To be specific, that 409Wh/kg figure is at the cell level. It's what you'd get for a battery in, say, gadgets or drones, where it'll likely see its first uses. In a car it would need surrounding structure, copper to wire all the cells together, and liquid cooling.

But because it's claimed to be so robust against heat – that's one of the great advantages of solid-state – the cooling can be simpler and lighter than in a conventional gel-cell lithium-ion battery.

"We are targeting 350Wh/kg at the pack level," Donut Lab’s CEO Marko Lehtimäki tells TopGear.com. That would mean a 500kg battery would have a capacity of 175kWh, vastly better than today's cars.

Donut Labs has previously shown results for its cell showing performance stability at 80 degrees C. In another test, after physical damage it still performed, although not at full capacity, but didn't catch fire. And it doesn't swell and shrink during charge and discharge as much as other solid-state designs, so Donut claims it doesn't need their weighty physical clamping.

Robust? Lehtimäki tells TopGear.com it can last 5,000 cycles when rapid-charged in this way even to 90 per cent capacity and beyond. If it's charged and discharged more gently, at more moderate temperatures, for example in grid energy storage, then he says 100,000 cycles would be possible.

Further, Lehtimäki repeats to Top Gear his earlier claim that there's no lithium or cobalt in the cells. Global supplies of these minerals are almost entirely processed in China, making battery production highly geopolitically vulnerable. He insists the minerals the Donut battery uses are instead cheap and available anywhere. "We could make all the batteries that the UK needs, in the UK, from materials out of UK, without any new mines."

The claims keep coming. Conventional battery cell factories demand medical-level dust-free environments. Lehtimäki says Donut's cells don't. "That means we can build these in brownfield facilities. Nothing will explode or whatever, and we wouldn't need special toxic or materials handling. We don't need clean rooms."

That's how Lehtimäki says these cells wouldn't cost any more than conventional lithium-ion cells, for all their immense claimed advantages in energy density, charge speed, durability and fire resistance. "We make these batteries out of materials that are not expensive and not volatile. Those are the two things that make cost."

I ask Lehtimäki what is in the cells, if not today's standard lithium and critical minerals. He refuses to say. Global giants have been chasing the Holy Grail of solid-state batteries for many years. So, he says, Donut needs to keep its recipe secret. "As a small company, we need to protect every bit of our intellectual property at application level, structure level, manufacturing level."

Huge Chinese companies, and others backed by Daimler and VW and Rimac, haven't yet got solid-state cells into production. Nor Nissan's or Toyota's own efforts. Donut is a tiny company, just 70 people including those working on the hubless in-wheel motor that gave the company its name. Even if taking cell tech from an external partner in Germany, it's amazing that they have achieved such a breakthrough… if they actually have.

In this information vacuum, many observers have been calling the Donut battery a hoax. Indeed the most advanced lithium-ion cells can perform, in charge speed and energy density, at the same level as Donut's claims. Several experts have speculated these Donut demo cells were actually lithium-ion all along.

Even the VTT report described the cells as "energy storage devices supplied by the customer, which the customer identified as solid-state battery cells". The customer being Donut.

Donut has also been criticised for the way it has concentrated on small and possibly gullible investors – who initially saw promise in the hubless motor – rather than tech-savvy venture capital.

Now though, Lehtimäki says he has proof they are in fact solid-state. Intertek, another testing organisation, carried out a test with a set of cells that had been made bipolar. This means each electrode is both an anode on one side and a cathode on the other (both poles), so they can be stacked in multiple layers. Like a club sandwich. This means you can build up a much higher total voltage than a single cell without the weight of wiring them in series externally.

Bipolar cells can't be made using conventional gel electrolytes, because the gel would leak around the electrodes and short the thing out. Intertek's report has for the first time shown photographs of a bipolar cell supplied by Donut, which Intertek dismantled after testing its storage. The layers are indeed dry.

Lehtimäki tells TopGear.com: "When you go from liquid electrolytes to solid states, that is a huge jump. But when you go from solid state to bipolar solid state, it's almost as big a jump. That's mainly on a packaging level, because that means that you're not chaining together hundreds of the small voltages anymore.

"The internal resistance is minimised, all the ions are travelling through each individual layer in the battery. You get rid of most of the heat generation and the copper and the bus bars. You don't need to monitor every cell, voltage and temperature, so you remove almost all the electronics from inside the pack."

But… there's a but. Deep in the 17-page Intertek report are the words: "Intertek have not verified that the device under test is a functioning battery cell which can be electrically discharged/charged."

There's no doubt mystery will surround the Donut battery for some time. They might be revealed a fraud. Or they might be a real thing. If they're real, while the exact mix of chemicals is complex, their origin and manufacturing are straightforward then the motivation for rivals to copy them will be huge, and Donut needs to keep secrecy to prevent that for as long as possible.

And while many battery experts say something this high-performing and cheap is basically impossible, many inventions are said to be impossible – until they aren't.