Mercedes Unveils Semi-Solid-State EV Batteries

Solid-state batteries remain several years from auto showrooms. The auto industry is pursuing the batteries, which replace liquid electrolytes with a solid ceramic or glass material, because of their potential to carry decisively more energy, charge faster and improve vehicle safety by reducing flammability over other types of lithium-ion batteries.

Now, Mercedes-Benz and solid-state battery manufacturer Factorial Energy have reached a hopeful halfway point, strapping semi-solid-state cells to the German automaker’s flagship electric sedan.

Mercedes announced it has begun road-testing the prototype cells in its EQS full-size sedan. Woburn, Massachusetts-based Factorial Energy supplies the pouch-format cells, and co-developed the pack with Mercedes-AMG High Performance Powertrains, the automaker’s Formula 1 racing subsidiary in Brixworth, U.K.

Mercedes says the battery delivers a real-world driving range beyond 1,000 kilometers (620 miles), about 25 percent farther than a conventional lithium nickel manganese cobalt oxide battery of the same size and mass. For comparison, the 2025 EQS 450+ is currently rated for 800 kilometers (497 miles) of range on Europe’s optimistic Worldwide Harmonized Light vehicles Test Procedure (WLTP) cycle, but just 627 kilometers (390 miles) on the U.S. Environmental Protection Agency’s more realistic estimates.

Mercedes and Factorial say the semi- (or “quasi-”) solid-state lithium-metal battery is the world’s first to make the jump from laboratory benches to a roadgoing car. Mercedes began conducting lab tests in Stuttgart at the end of 2024 before embarking on street testing in February.

“Being the first to successfully integrate lithium-metal solid-state batteries into a production vehicle marks a historic achievement in electric mobility,” says Siyu Huang, CEO and co-founder of Factorial Energy.

Mercedes-Benz Tests Semi-Solid Batteries

Mercedes

Huang says the FEST cells (for “Factorial Electrolyte System Technology“) feature a robust energy density of 391 watt-hours per kilogram. That compares with 300 Wh/kg or less for today’s leading high-nickel cells. The 106 amp-hour cells feature a solid electrolyte infused with gel or liquid. Factorial is also developing an all-solid-state “Solstice” cell with a sulfide electrolyte that targets up to 500 Wh/kg at the cell level, for up to 80 percent longer driving range. That 500 Wh/kg is double that of many high-nickel batteries, and about 2.5 times as good as that of the best lithium iron phosphate batteries.

“That’s an upper limit of where solid-state could go,” Huang says.

So automakers could potentially cut today’s battery packs roughly in half—in size, weight and capacity—while delivering identical driving range; or keep packs roughly the same size while dramatically boosting range or performance. Sharply downsized packs would create a chain of engineering gains including lighter chassis, suspensions, cooling systems, and other components to support batteries, as well as cost savings in packaging and space for occupants and cargo.

Initially, Huang believes the tech will be best suited to premium and high-performance cars, as a superior alternative to high-nickel cells. The company also sees potential for consumer devices, drones, or marine applications with serious power-discharge demands.

Just as important as performance gains, the FEST cells are compatible with existing lithium-ion battery manufacturing, according to Huang, which saw 846 gigawatt-hours of cells deployed globally in electrified cars in 2024. Factorial says its processes would allow battery makers to use roughly 80 percent of existing equipment. Huang estimated a battery maker could convert a 1-gigawatt factory to quasi-solid-state production for as little as US $10 million.

“So it’s much easier to ramp up, not only for manufacturing, but in yield improvements and cost reduction,” Huang says.

The Factorial product development team use an insulator in the lab.Factorial Energy

Factorial’s Manufacturing Footprint

Factorial currently has a manufacturing footprint for 500,000 FEST cells annually, and is seeing battery yields of 85 percent—solid performance for a pilot facility, though Huang says a commercial manufacturer would need to see a roughly 95 percent yield to make cells profitably. The company, which raised $200 million in a funding round led by Mercedes and Stellantis, is also supplying FEST cells for a demonstration fleet of Dodge Charger Daytonas. Those electric muscle cars are expected to hit streets in 2026. The Charger Daytona is among several electrified models from Alfa Romeo, Chrysler, Dodge, Jeep, and Maserati to be built on Stellantis’ STLA Large platform.

The quasi-solid-state design also allows for a lithium-metal anode that replaces graphite and carries more energy. The Mercedes battery is also notable for its patented “floating cell carrier.” Those pouch cells expand during charging, and contract during discharge. Mercedes’ F1 division developed a flexible carrier to manage those volume changes, with pneumatic actuators to help maintain proper pressure inside cells.

“There’s like a ‘breathing behavior’ during the cycling of the cell,” Huang says. “So you need a system to accommodate that breathing and make it more efficient over its lifetime, while still maximizing energy efficiency.”

As for its all-solid-state Solstice batteries, Factorial has developed a 100 percent dry-cathode process that can eliminate all hazardous solvents in cathode coating. Those carcinogenic solvents currently require energy-intensive evaporation to safely recycle them. The design also bypasses the energy-sapping “formation” process, in which freshly assembled cells must be charged and discharged to activate materials and create a stable electrolyte layer on electrodes. Together, the dry coating and cell design promise more sustainable battery production with fewer costs and environmental impacts.

Factorial’s batteries are among several promising solid-state developments. Volkswagen has partnered with Silicon Valley’s QuantumScape. Colorado’s Solid Power has a joint development deal with BMW. Honda recently unveiled a demonstration solid-state production line in Sakura City, Japan. Toyota is also developing solid-state cells in-house, part of a $13.6 billion investment in battery R&D.

Like some experts, Huang sees 2030 as a viable target for a tech that could make EVs go farther, charge faster and become more affordable.

“I think there’s potential to see these batteries in showrooms by the end of the decade,” Huang says.

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