When Billy Woodford and his friends set out to build a new kind of battery that could replace a coal plant, their breakthrough technology took inspiration from the disposable hand-warming sacks that spectators use at football stadiums. The battery’s key ingredient: rust.

Woodford’s startup, Form Energy Inc., went into business in 2017 with a clear and ambitious goal. The battery needed to soak up renewable energy during the day and release it at night, and to keep running after sunset and on windless days. A lithium-ion battery, like those in EVs and smartphones, would work, but no utility can afford to run that type of battery on the grid for more than a few hours. Utilities need one that will also be cheap enough to deploy for 100 hours or more. “You can have pretty much any battery do any duration on the grid,” says Mateo Jaramillo, who worked at Tesla Inc. before co-founding Form and is today its chief executive officer. (Woodford is chief technology officer.) “It always comes down to cost.”

Batteries are a notoriously tricky technology to master. Bringing a new chemistry of materials from the lab to a commercial application takes an average of 15 years, too long to help the US reach its goal of having a carbon-free grid by 2035. Woodford and his team, however, built their battery in record time: four years to the first pilot and seven years to the first commercial deployment. Their process starts with low-cost materials and iterates from there, and it shows that innovation can pay off in the limited time available to meet global climate goals.