Solid state batteries are inching closer to reality as BMW has become the latest automaker to introduce a road-going prototype. It’s based on the i7 and features high-tech cells from Solid Power.

The companies have been working together since 2016 and the prototype will be driven around Munich as part of the testing process. The automaker didn’t say much about this, but stated they’ll be examining cell expansion, operating pressures, and temperatures. They added, the “use of Solid Power cells with sulfide-based electrolytes and their complete integration into a battery pack will provide the BMW Group with further important findings in the test program over the coming months.”

More: Mercedes EQS Solid State Prototype Hits The Street With 621 Miles Of Range

While BMW was coy on specifics, solid state batteries offer an assortment of advantages over traditional lithium-ion batteries. One of the most notable is higher energy densities, which can allow for smaller battery packs or longer ranges without weight penalties. Solid state batteries also promise to deliver improved performance and safety.

BMW didn’t say when we can expect to see solid state batteries in production vehicles, but implied they’re still a ways off. As they explained, “further development steps are required to implement ASSB [all-solid-state battery] technology in a competitive overall storage system.”

That being said, the company is optimistic about the future. BMW battery guru Martin Schuster said, “Our i7 ASSB test vehicle on the road is a perfect example of the BMW Group’s technology-open mindset. We are continuously advancing the development of new battery cell technologies and are constantly expanding our know-how with valuable partners such as Solid Power. ”

Besides BMW and Mercedes, Stellantis is working on solid state batteries and has plans to introduce a fleet of prototype Dodge Charger Daytona EVs by the end of the year. Like the EQS, they use Factorial cells and these are slated to have an energy density of 375 Wh/kg.