In 2010, a pair of MIT materials scientists helped launch 24M, promising to deliver cheaper, better batteries by stripping out inactive materials in the electrodes.

Eight years later, you still can't buy the startup's products. But in an interview last week, chief executive Rick Feldt said the "semisolid" lithium-ion batteries built in the company's pilot lab have leapfrogged those on the market today in terms of energy density. 24M will begin working with an industrial partner next year to develop a small commercial plant and hopes to deliver its first products in 2020—five years past the company's original time line.

Higher energy density means batteries cost less, weigh less, and last longer, promising electric vehicles without the sticker shock or range anxiety, or phones that don't demand an extra battery pack to get through the day.