SK On and Factorial team up on solid-state cells: 375 Wh/kg and a charge in 18 minutes

Solid-state batteries have come close to production cars, but affordable electric models will have to wait several more years. SK On has started up a pilot line, Factorial is expanding output and running its cells in real vehicles, and on July 29 the two companies agreed to do the job together. It is still too early to mistake these steps for the full-scale output of millions of packs.

South Korea’s SK On has built a pilot plant in Daejeon covering roughly 4628 square meters. It turns out experimental sulfide cells and lithium-metal batteries and checks their durability, safety and manufacturing stability. The first target is a volumetric energy density of 800 Wh/l, with 1000 Wh/l to follow. Commercialization has been pulled forward from 2030 to 2029.

Factorial has gone further in vehicle testing. Its FEST cells sit in a Dodge Charger Daytona development car built on the STLA Large platform, where Stellantis is checking charging, reliability and how the pack behaves under heavy load on public roads. In the lab the 77 Ah cells held 375 Wh/kg over more than 600 cycles, charged from 15% to 90% in 18 minutes and worked from −30 to +45 degrees Celsius. In the autumn of 2025 a modified Mercedes-Benz EQS running on Factorial cells covered 1205 km on a single charge.

FEST cannot be called a fully solid-state battery without reservations: a small liquid component remains in the electrolyte. That design makes the switch to the new technology easier — according to Factorial, its cells are 80% compatible with existing lithium-ion production lines.

That compatibility is exactly what the new alliance is built on. On July 29 Factorial and SK On signed a memorandum of understanding: they will assess whether the Korean manufacturer’s existing lithium-ion plants can produce FEST cells. The scale is considerable — more than 200 GWh of annual capacity worldwide, around 100 GWh of it in the United States. “A battery breakthrough only matters if it can be manufactured at scale,” said Factorial chief executive Siyu Huang. The document is non-binding for now, and the terms of the partnership are to be set out in separate agreements.

Factorial’s Korean site in Cheonan is rated at up to 200 MWh. The company says it has produced thousands of large-format cells and, at the A-sample stage, shipped Mercedes-Benz more than 1000 cells rated above 100 Ah. The expansion of the existing lines in South Korea and the United States is due to be completed by the end of 2027, and Factorial does not intend to build new plants — the bet is on somebody else’s capacity. The first batches are meant mostly for expensive cars, aviation and drones.

The decisive milestone, then, will not be another energy density record but proven battery life, consistent quality across thousands of cells and the move from megawatt-hours to gigawatt-hour output. Until that happens, solid-state batteries will remain largely a technology for prototypes and low-volume models.