The agreement pairs Factorial’s fabless cell development with Mitsui Kinzoku’s sulfide electrolyte expertise in all-solid-state batteries
(Image courtesy of Factorial Energy)
Mitsui Kinzoku and Factorial Energy have signed a joint development agreement to support industrialisation of Factorial’s Solstice all-solid-state battery platform. Factorial stays fabless, leading cell design, process development, supply chain qualification, line integration and manufacturing validation, while Mitsui Kinzoku, a leading producer of sulfide-based solid electrolytes, contributes materials and manufacturing expertise. Solstice is a separate platform from the FEST cells used in the Mercedes-Benz EQS and Dodge Charger Daytona vehicle programmes.
The electrolyte expertise behind Factorial’s all-solid-state push
Factorial announced the agreement in September and describes Mitsui Kinzoku as one of few companies worldwide with foundational sulfide solid electrolyte technology. The release also credits Mitsui Kinzoku with an estimated 90% share of the semiconductor market for ultra-thin copper foil. That materials science expertise, it says, underpins A-SOLiD, the company’s sulfide solid electrolyte. A-SOLiD was unveiled in 2016 and is advancing toward mass production at a dedicated plant in Saitama, Japan.
Mitsui Kinzoku’s own notice from September 2024 describes A-SOLiD as an argyrodite-type sulfide solid electrolyte. The company said then that a new plant for initial mass production of A-SOLiD in Ageo, Saitama, would start operating in 2027.
Solstice, as Factorial describes it, is a sulfide-based all-solid-state platform with high energy density and improved thermal stability. Proprietary dry-cathode processing, the company says, enables more cost-effective manufacturing. Factorial says the collaboration builds on strategic investments from IQT and POSCO Future M and its June Nasdaq listing.
All-solid-state Solstice and quasi-solid FEST cells
The road-testing milestones in Factorial’s company background belong to FEST, a separate platform. Mercedes-Benz, for example, said its lightly modified EQS used lithium-metal cells from Factorial based on FEST technology. That car completed a 1,205 km drive without a charging stop at the end of August 2025. Similarly, Stellantis and Factorial announced the integration of FEST technology into a Dodge Charger Daytona development vehicle in June.
On its technology page, Factorial describes FEST as combining a lithium-metal anode, quasi-solid electrolyte technology and a high-capacity cathode. Solstice, by contrast, uses a sulfide-based all-solid-state electrolyte material, and it is the platform the Mitsui Kinzoku agreement targets.
Looking ahead, Factorial expects the partnership to accelerate the global scale-up of Solstice. In addition, the release places its expectations for manufacturing capabilities, supply-chain development and production readiness within forward-looking statements. Chief executive Siyu Huang said battery innovation depends on the whole supply chain and, increasingly, on an industry coalition.
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