Use a gadget with a lithium-ion battery inside and you'll eventually learn that these power packs decay once you've cycled them enough times. But have you ever wanted to see direct evidence of why they have a limited lifespan? The Department of Energy is happy to oblige. It developed a special device that, when placed inside an electron microscope, lets it take nanoscale pictures of lithium-ion cells as they drain and charge. As you can see above, lithium (the black fluff in these photos) temporarily deposits on electrodes during each cycle, but doesn't completely dissolve. The more you use a battery, the more permanent deposits you get and the less capacity you have.