The start-ups hoping to return battery making to the US

As heatwaves scorched Europe this summer, workers at a US battery company noticed an uptick in demand.

San Diego-based Unigrid sells a nine kilowatt hour battery for people's homes. Sweltering European homeowners were apparently looking for ways of storing power when it was available from the grid at lower rates. They could then run air conditioners more cheaply, off the battery, during the worst heat of the day.

"We have requests coming from Spain, France, Germany, The Netherlands," says Darren Tan, co-founder and chief executive. "We've shipped the first 100 units and [expect to ship] 1,000 before the end of the year."

Unigrid's battery is an emerging technology – it's a sodium ion, rather than lithium ion device. Sodium ion batteries have got energy storage experts excited because they could end up much cheaper to build and operate than lithium ion.

They're potentially less likely to catch fire and should also work in a wider range of temperatures, even down to lows of minus 40C , external .

But there's a debate over what constitutes the best, and safest, sodium ion battery design. Plus, more than one , external US company working in this space has floundered in recent years , external .

China leads the world in sodium ion battery manufacturing at present so US competitors are going to have to work hard to keep up.

The prize that awaits is the possibility of batteries made in the US with 100% US-sourced materials. This is unlike lithium ion batteries, where China dominates the materials , external supply chain as well as manufacturing.