Yes, our bark-skinned friends are nice and beautiful and we shouldn't mess with them too much. But here's the thing: we already chop them down for paper, so why not use their spare woody meat for batteries too? Like previous attempts at organic energy storage, it all hinges on mimicking photosynthesis. Up to the a third of the biomass of a tree is a pulpy substance called lignin, which is a by-product from paper production and which contains electro-chemically active molecules called quinones. With a bit of processing, Professor Olle Inganäs at Linköping University in Sweden reckons he can turn lignin into a thin film that can be used as cathode in a battery, and he believes it's efficient enough to start industrial-style development of the technology. "Nature solved the problem long ago", he says, and "[Lignin is] a source that never ends". Meanwhile, if you imagine Inganäs as having a long white beard and cloak, then, er, snap.