The problem at the heart of many sustainable-energy systems: How to store power so it can be delivered to the grid all the time, day and night, even when the wind's not blowing and the sun's not shining? At MIT, Donald Sadoway has been working on a grid-size battery system that stores energy using a three-layer liquid-metal core. With help from fans like Bill Gates, Sadoway and two of his students have spun off the Liquid Metals Battery Corporation (LMBC) to bring the battery to market.