When MIT chemistry professor Don Sadoway received an email from Bill Gates's personal assistant in 2009, asking if he would meet with Gates, Sadoway assumed it was a prank, and ignored it. It wasn't until he got a follow up email, this time from Gates, himself, that he realized this was no joke. As it turned out, the billionaire had been taking some of Sadoway's free chemistry courses online. Gates wanted to talk to him about a novel concept Sadoway was working on to develop a new type of battery that could make renewable energy a practical, affordable mass market option, once and for all.

The big problem with adopting solar or wind energy at a mass scale is the fact that there's no affordable way to store that energy for the grid when the sun's not shining and the wind's not blowing.

Sadoway's invention is designed to solve that problem. "We sat in my office for 90 minutes," Sadoway says of his initial meeting with Gates. "He said, 'If you ever decide to spin this thing out as a startup, let me know. I'd like to put some money in.' "

Sure enough, in 2010, backed by Gates and French energy company Total, Sadoway and then-Ph.D. candidate David Bradwell launched Ambri to develop a liquid metal battery that's both cheaper and longer-lasting than existing alternatives, like lithium-ion batteries. Each battery is made up of two types of liquid metal, one low-density metal that floats on top and one high-density metal that sinks to the bottom, with a layer of salt in between. The benefits are huge: The materials themselves are abundant and inexpensive. And the batteries can withstand years of use without degrading--after 10 years they'll still operate at more than 99.5 percent capacity--which leads to maintenance and replacement cost savings.

Ambri, which now has 40 employees and is led by CEO Phil Giudice, is also backed by Khosla Ventures. If all goes according to plan, the team will begin distributing its batteries, which come stacked on racks inside a 40-foot shipping container, to three pilot clients by next year. "In 10 years, if this continues going the way it is right now, it could change the world," says Sadoway. "It's the last piece of the puzzle."