When batteries die what’s left over is a mess. Devilish “black mass” is a shredded, toxic mix from the insides of battery cells that have reached the end of their useful lives. “That’s about as close as we go,” warns Benjamin Wickham, director of process chemistry at startup Altilium Metals. I had come within prodding distance of a tonne of the stuff at the battery recycling company’s test lab in Tavistock, on the edge of the Dartmoor national park. That dark powder, and the valuable metals within it, will play an increasingly important role in decarbonising Britain’s economy over the…