In January, a Finnish-Estonian startup proclaimed it had developed a truly solid state battery, a holy grail for the technology industry. Donut Labs’ cell wasn’t just solid state, however. It claimed it was made from cheap and easily available materials, would charge to full in a few minutes and last for hundreds of years. If real, such a device would change the face of the world, which is why plenty of people don’t think it is. And, as the company makes more effort to demonstrate it is telling...