Factorial Solstice solid-state battery: 450 Wh/kg and scaling with Mitsui Kinzoku
Factorial will work with Mitsui Kinzoku to scale Solstice, its sulfide-based all-solid-state battery platform with potential specific energy of up to 450 Wh/kg.
Up to 450 Wh/kg — that is the potential specific energy Factorial claims for its all-solid-state Solstice platform. The US company has now brought in Mitsui Kinzoku, a producer of sulfide solid electrolytes, to help scale it.
The key here is not the agreement itself, but the material. Mitsui Kinzoku developed the A-SOLiD sulfide electrolyte and is already expanding capacity in Saitama. According to the company’s official information, its initial mass-production plant is planned to start operation in 2027. Factorial is therefore linking its cell architecture with a partner that is simultaneously building an industrial base for electrolyte production.
For Factorial, this is the next stage after moving the technology into vehicle testing. In June 2026, Stellantis integrated FEST cells into a prototype Dodge Charger Daytona and began road tests. The figures of 375 Wh/kg, charging from 15% to 90% in 18 minutes and operation from −30 to +45 °C, however, come from FEST cell validation results that Stellantis officially published in 2025.
Solstice is Factorial’s separate all-solid-state platform based on a sulfide electrolyte. The company has cited potential specific energy of up to 450 Wh/kg and the ability to operate at temperatures above 90 °C. In 2024, Factorial said it had scaled Solstice A-sample cells to 40 Ah.
The main limitation remains the same: high specific energy alone does not mean a technology is ready for mass automotive production. The industry is now tackling the scaling problem itself. Geely is preparing a similar move from development to pilot deployment, with solid-state battery tests in vehicles planned for 2027. Factorial’s partnership with Mitsui Kinzoku matters primarily because it adds a real supply chain for a key Solstice material.