Solid-State EV Battery: ProLogium, Rimac Set 2028 Road Tests

A mockup in Paris and a dated roadmap

ProLogium and Rimac Technology used the 2026 Paris Motor Show to show their joint solid-state battery work for the first time. What they displayed was a concept mockup of a pack, not working hardware. ProLogium, the Taiwanese cell maker, said the two companies have finished a concept mockup of a battery pack built on a pouch cell-to-pack design. The project started with a memorandum of understanding signed in September 202516. Next comes a functional battery segment, which will serve as the test platform for later work13.

The more useful part of the announcement is the timeline. Rimac Technology says the functional segment will appear at CES 2027, a complete working pack will follow in the second half of 2027, and on-road vehicle testing should start in the second half of 202814. Automotive World reported the same sequence12.

The companies describe the split of work this way. ProLogium supplies the cells, and Rimac, the Croatian Tier 1 supplier owned by Rimac Group, handles system development and integration14. The stated aim is to take the project from a concept toward real hardware one step at a time13.

Integration, not chemistry, is now the bottleneck

The coverage agrees on one point: at this stage, building a working pack is harder than improving the cell. Automotive World put it plainly, saying the roadmap shows integration has overtaken chemistry as the factor that sets the pace for solid-state batteries12. Electrive made a similar argument. High energy density in a single cell does not automatically produce a lighter or smaller vehicle battery. Pack design, cooling, mechanical mounting and the safety concept decide how much of that advantage reaches the car14.

Rimac's own description supports this. Daniel Kišić, the company's principal product architect, said it is testing a wide range of integration approaches side by side on functional segments. The work covers thermal management, cell swelling, how cells are mounted, and overall system design. Some versions aim for maximum energy density and less weight and space, while others aim for performance13.

Swelling is the detail to watch. Automotive World noted that managing it is a practical obstacle for the solid-state pouch cells Rimac is designing around12. The same outlet also pointed out that road tests are more than two years away, so any production use is further off still12. That is a realistic reading, and it is borne out by what Rimac said a year earlier. In October 2025, Rimac COO Nurdin Pitarević told Automotive News Europe that the partners wanted a first sample by the end of 2027, a B-sample ready for in-vehicle testing by 2028, and a battery ready for small-scale production by 203017. The Paris roadmap matches that plan, which suggests the program is on schedule but is not moving faster. Analysts should treat 2030 small-batch production as the earliest realistic commercial date, not 2028.

The partners frame the benefit in terms automakers will recognise. If the pack is validated, its weight savings could be used for more energy at the same mass, a smaller pack with the same range, or some mix of the two14. Pitarević has given more concrete targets. He said automakers want roughly a 100 kWh pack with 500 to 600 km of range. The partners are aiming for 10-to-80 percent charging in under 10 minutes and energy density 20 to 30 percent above rivals, helped by a composite pack frame instead of steel or aluminum17. He also said a year ago that each side was about halfway to its goals17.

What ProLogium's cells promise, and what has been verified

ProLogium's claims about its cell technology are ambitious, and it helps to separate company statements from third-party test results. The company calls its fourth-generation chemistry a "Superfluidized All-Inorganic Solid-State Lithium Ceramic Battery." It pairs an inorganic electrolyte with an all-ceramic separator and an all-silicon anode1. ProLogium claims up to 860 Wh/L volumetric density and room-temperature ionic conductivity of 57 mS/cm, which it says is about five times that of conventional liquid and sulfide electrolytes2. It also claims 60-80% recharge in four to six minutes, and says the cells need no high external pressure, which simplifies pack design1.

That last claim matters most for the Rimac project. The original MoU promised a module-free architecture and high packaging efficiency "with no need for pressurized modules"11. Many solid-state designs need heavy pressure fixtures that cancel out much of their energy-density gain. If ProLogium's cells really don't need them, Rimac's cell-to-pack approach becomes far more plausible. Even so, Rimac is still treating swelling as an open engineering problem13, so the pressure question looks managed rather than solved.

Some numbers have outside verification. ProLogium says TÜV Rheinland confirmed 360 Wh/kg for a recent cell, and that UL Solutions ARC testing found no thermal runaway under the Heat-Wait-Seek method6. Electrek later reported a TÜV result of 381 Wh/kg for a 185.4 Ah large-format Gen 3.5 cell, which the company says is now in mass production in Taiwan. The 400 Wh/kg figure ProLogium gives for Gen4 remains a company claim10. So the numbers increase with each generation, and the newest ones are the least independently tested.

A widening network of partners and capital

The Rimac update is one piece of a broader commercial push. In late September, Mercedes-Benz signed a joint test agreement that gives it priority access to Gen4 cells. Mercedes will test electrical, thermal and safety performance, extending a relationship that dates to 201610. Electrive stressed that Mercedes has made no decision on series production14. ProLogium is also working with FEV, which showed a module concept at CES 2026 aimed at about 1,000 km of range7. These are parallel pack-integration efforts, which suggests ProLogium is spreading its system-engineering risk across several Tier 1 and engineering partners instead of relying on one.

Money is the other key factor. In May, ProLogium agreed to list on Nasdaq through a merger with Translational Development Acquisition Corp, at an expected pre-money valuation of about $3.8 billion. The deal is meant to fund Gen4 scale-up and the Dunkirk gigafactory, which is backed by a French subsidy package of up to about €1.4 billion6. Electrek noted that ProLogium follows QuantumScape, Factorial Energy and Solid Power in using a SPAC to go public9. In practice, this makes milestones like the Paris mockup matter more to investors, and readers should weigh announcements with that in mind.

Dunkirk timelines don't line up

This is where the coverage diverges most. Earlier ProLogium materials said Gen4 mass production in Dunkirk would begin in 2028, at an initial 0.8 GWh1. The SPAC announcement pushed the dates back: ramp-up between Q4 2028 and Q1 2029, with full mass production and deliveries in Q2 20296. Automotive News Europe had already reported, citing French media, that the plant's start had slipped by a year17. Long-term capacity figures vary as well. ProLogium has mentioned land reserved for up to 48 GWh2, while Electrek and Electrive cite 44 GWh1014. The direction is clear even though the figures differ: Europe's most visible solid-state factory is moving forward, but more slowly than first promised. Given Northvolt's collapse, which Automotive News Europe cited as background17, a slow start that actually happens is a better result for European supply chains than a fast one that fails.

The stationary-storage angle

For clean energy more broadly, the most interesting signal may not be the cars. At CES 2026, ProLogium said it would expand its platform into construction machinery, e-bikes, humanoid robots, and power management and energy storage systems for AI data centers2. Through a separate MoU with power-electronics company Delta, the two will first design 48V battery modules using existing cells and then develop prototype systems22. ProLogium's SPAC partner presented the company as a source of high-performance distributed energy, naming data centers alongside aerospace and robotics6.

This needs some caution. Nothing reported points to utility-scale grid storage, a market where low cost per kilowatt-hour usually matters more than energy density. The more likely early fit is safety-critical, space-limited storage such as data-center backup power, where a cell that resists thermal runaway is worth paying for. If ProLogium's claims about avoiding rare elements and keeping costs close to liquid-electrolyte cells hold up22, a move toward larger stationary systems would become plausible. That is a hypothesis to test against real deployments, not a conclusion.

The bottom line

The Paris reveal is a modest step. The stakes behind it are much larger. The main question for solid-state batteries has shifted from whether a cell can perform to whether a whole pack can survive automotive validation and be built at an acceptable cost. ProLogium and Rimac have set public checkpoints for CES 2027, late 2027 and late 2028. Hitting them on schedule would be a better test of the technology than any lab result.