@interestingengineering: In August 2025 a Mercedes-Benz EQS drove 1,205 kilometres from Stuttgart to Malmö on a single charge and arrived with roughly 85 miles still in the pack. It was running a solid-state battery, the technology Toyota, Volkswagen, Samsung SDI, CATL and BYD have collectively put more than $20 billion into. Yet as of today, not a single production car ships with a true all-solid-state cell. We break down how the technology actually works, the three failure modes that have kept it stuck, who is genuinely close to building it, and why semi-solid cells already on Chinese roads may matter more in the short run.