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…