Since announcing earlier this year that it was on the cusp of a major battery breakthrough, Finnish startup Donut Lab has faced a lot of questions, and plenty of skepticism, about its production-ready, solid-state battery. Could the company really make a fast-charging battery at scale while avoiding some of the theoretical production headaches that have stymied past efforts? Today, Donut Lab sought to dispel some of the doubts with the release of the first independent test of its battery, evaluating its charging speed and the “thermal behavior” of its pack.
The test, which was conducted by state-owned VTT Technical Research Centre of Finland, concludes that the battery is able to be charged significantly faster than a traditional lithium-ion battery. In several tests, the battery was able to charge from 0–80 percent in around 9.5 minutes, while retaining 100 percent of its capacity. In other tests, 0–80 percent was achieved in about 4.5 minutes while retaining 99 percent capacity.
Solid-state batteries, which are often referred to as the “holy grail” of batteries, have eluded researchers for decades. Most EV companies use “wet” lithium-ion batteries, which use liquid electrolytes to move energy around. But these batteries can be slow to charge, can freeze up in subzero temperatures, and contain flammable material that can be hazardous in the event of a crash. Solid-state packs are made of “dry” conductive material that can hold more energy without any of the thermal runaway problems of a traditional battery.
Usually, if you force-feed a battery that fast, the chemistry degrades instantly and you lose capacity. According to these tests, Donut Lab’s battery kept almost 100 percent of its energy potential even after being blasted with power. That could transform an EV charging session from a 30–40 minute sojourn into a 5–minute visit, akin to a gas station refuel.
That could transform an EV charging session from a 30–40 minute sojourn into a 5–minute visit, akin to a gas station refuel.
Most EV batteries, like Tesla or Hyundai, have active cooling systems that keep the battery cool while charging. In this test, VTT stripped all that away, using a passive cooling system in order to prove that the battery wouldn’t require a heavy, expensive liquid cooling system to charge safely. They did this two different ways, first by sandwiching the battery between two pieces of aluminum, and second by placing the battery on just one piece of metal — all while monitoring the temperature.
VTT used “C-rate” measures to describe how fast a battery charges relative to its capacity, where 1C represents charging from empty to full in one hour. Traditional lithium-ion batteries typically charge at 1C to 3C with active cooling, whereas in this measurement the charging power rises to significantly higher rates without active cooling. VTT started with a standard discharge capacity test at 1C, followed by rapid charging tests at 5C and 11C with both cooling configurations.
Under these conditions, Donut Lab’s battery was successfully charged at 5C for over nine minutes. At this charging power, the battery cell reached an 80 percent state of charge in about 9.5 minutes and a full 100 percent state of charge in just over 12 minutes. When discharged after charging, 100 percent of the charged capacity was available from the cell.
There were some hiccups. During one test with just one cooling plate, the battery got so hot it hit the 90 °C safety limit, and the machine automatically shut off to prevent damage. The researchers realized the battery wasn’t touching the cooling plate tightly enough, so they strapped it down to improve thermal contact and the heat could escape into the metal more efficiently.
“Unlike other solid state batteries requiring high compressive pressures and undergoing volume changes of up to 15-20% during recharging cycles, the Donut Battery does not require special compression or more extensive cooling,“ Donut Lab CTO Ville Piippo said in a statement. “This greatly simplifies the structure of battery packs and enables solutions that are cost-efficient, powerful, and better than traditional lithium-ion batteries in terms of energy and power density.”
Donut Lab has claimed that its battery delivers 400 watt-hours per kilogram. By comparison, most lithium-ion batteries today range from 200 to 300 Wh-per-kg. The company also says the battery can charge in under 10 minutes and last 100,000 cycles. Current lithium-ion batteries typically last between 1,500 and 3,000 cycles. The “Solid State Battery V1” that VTT tested has 26Ah of capacity and 94Wh of nominal energy, according to the institute.
But before you go popping the champagne, keep in mind that the report doesn’t address some key questions regarding Donut Lab’s solid-state battery. First, VTT doesn’t confirm the chemistry of the pack, but simply takes it at face value. Second, the test didn’t address what Verge contributor Tim Stevens refers to as the “dendrite issue,” in which microscopic stalagmites growing from anode to cathode across the solid-state electrolyte can cause electrical shorts. Perhaps Donut Labs will address that in subsequent independent test results, which it is promising as part of its “I Donut Believe” series.
[Notigroup Newsroom in collaboration with other media outlets, with information from the following sources]
