Rahul Somvanshi
The gap between solid-state battery technology and commercial vehicles has long been a manufacturing problem rather than a chemistry one. A new partnership announced September 17, 2026, directly targets that gap.
Massachusetts-based Factorial Energy and Japan’s Mitsui Kinzoku signed a joint development agreement on September 17, 2026, to industrialise Factorial’s Solstice all-solid-state battery platform. The partnership connects Factorial’s cell design and manufacturing validation expertise with Mitsui Kinzoku’s A-SOLiD™ argyrodite sulfide solid electrolyte, which is advancing toward mass production at a dedicated facility in Saitama, Japan. Factorial operates as a fabless battery developer — it designs cells and validates manufacturing processes rather than building its own production plants.
Solstice targets an energy density of 450 Wh/kg, approximately 80 percent higher than traditional lithium-ion cells. Mercedes-Benz completed real-world road testing in an EQS test vehicle that covered more than 1,200 kilometres — roughly 745 miles — on a single charge using Factorial cells. Stellantis integrated Factorial cells into a Dodge Charger Daytona demonstration vehicle.
If you’re waiting for a solid-state EV battery before buying electric, this deal moves the needle — but not to your driveway yet. Factorial’s Solstice platform claims 450 Wh/kg, nearly double today’s lithium-ion cells. The Mitsui Kinzoku partnership is about scaling manufacturing, not launching a product. Expect commercial vehicle applications before passenger cars.
Why the Mitsui Kinzoku deal matters
The deal divides the challenge of solid-state battery manufacturing into two explicit responsibilities. Mitsui Kinzoku handles the chemistry: its A-SOLiD™ electrolyte combines the high ionic conductivity needed for power output with the physical processability needed for industrial manufacturing lines. Factorial handles the engineering: cell design, process development, supply-chain qualification, line integration and manufacturing validation.
One detail competitors have largely missed: Mitsui Kinzoku controls an estimated 90 percent of the global market for ultra-thin copper foil used in semiconductor manufacturing. That level of market concentration reflects precision manufacturing infrastructure and materials expertise that Mitsui Kinzoku has built over decades.
Solstice also maintains thermal stability up to 90°C (194°F), a specification relevant to hot-climate markets and to vehicle safety in high-heat scenarios. See also: how other solid-state battery makers are advancing the race to 700+ Wh/kg.
Solstice uses dry-cathode processing — a manufacturing approach that eliminates liquid solvents from cell assembly. This has implications for production cost and environmental footprint at the factory level.
Factorial’s structure as a fabless developer is the operationally distinctive part of this arrangement. The company outsources electrolyte synthesis to Mitsui Kinzoku’s Saitama plant and validation to established manufacturing partners, rather than building its own greenfield production facilities.
When will solid-state batteries be available in electric cars?
Factorial has targeted commercial deployment of its Solstice platform, with the Mitsui Kinzoku partnership aimed at supporting industrialisation toward that goal. First integration is expected in performance and commercial vehicle applications. Mass-market production requires manufacturing partnerships to bring per-cell costs down before wider availability becomes practical.
What is the difference between solid-state and lithium-ion batteries?
Solid-state batteries replace the liquid electrolyte in conventional lithium-ion cells with a solid material — in Factorial’s case, a sulfide-based electrolyte. This removes the flammable liquid component, allows higher energy density, and improves thermal stability. The manufacturing challenge is producing solid electrolytes consistently at the volumes needed for vehicle production.
What does Mitsui Kinzoku bring to Factorial?
Mitsui Kinzoku supplies its A-SOLiD™ argyrodite sulfide solid electrolyte, advancing toward mass production at a dedicated facility in Saitama, Japan. The company holds an estimated 90 percent market share in ultra-thin copper foil for semiconductors — a supply-chain position that reflects its precision manufacturing capacity and materials expertise.
Factorial has also confirmed its first commercial passenger-vehicle order and a first aerospace order earlier in 2026. Field test telemetry from Stellantis’s Dodge Charger Daytona demonstration fleet is expected in early 2027. Check back for production timeline updates.
