US battery maker Unigrid sees rising European demand for sodium-ion home batteries amid heatwaves

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.

For Tan, China has helped his company get to market. The Unigrid battery currently uses sodium ion cells made there, rather than in the US. "We can switch back and forth and choose what battery we want to work with," says Tan, explaining that, in future, his firm might source cells from the US – though these may come at a different cost.

There's another consideration. Unigrid's battery contains chromium-3 in its cathode. In a battery, electrons flow from cathode to anode, external when charging and in the opposite direction when discharging or providing power.

Prof Dame Clare Grey at the University of Cambridge, who is co-founder of battery firm Nyobolt, points out that under certain conditions chromium-3 may form chromium-6, a toxic, cancer-causing material., external

"If you're going to go for sodium […] and you're going to sell safety, then don't start putting elements in that are going to raise red flags," she tells the BBC.

Ivana Hasa at the University of Warwick, who has no connection to Unigrid, agrees that it is possible for chromium-3 to form chromium-6 at very high states of charge, though she says this can be mitigated by how voltage is controlled within the battery. "Safety tests reported on the cell chemistry are very promising," she says of Unigrid's tech.

Tan says concerns around toxicity of the battery's materials are "overblown" and emphasises that Unigrid's battery has passed certification and safety tests for sale in Europe and is currently undergoing such evaluations for the US market.

Third-party tests have not been able to get the battery to form chromium-6, he says. "We've tried very hard to try and produce it, we've overcharged it as much as we can... We were unsuccessful in generating chromium-6."

The choice of cathode material, or how the electrolyte – which separates cathode and anode – is formulated are key details influencing the performance and longevity of sodium ion batteries. Some manufacturers are not using chromium for their cathode but hard carbon instead, or sodium iron phosphate pyrophosphate (NFPP).

In general, confidence around sodium ion batteries' reliability has increased during the last few years, says Prof Dame Grey, meaning they are now finding their way into grid storage applications.

Last December, LFP batteries – a relatively cheap form of lithium ion battery – were trading at $81 per kilowatt hour, external (kWh) whereas Unigrid's sodium ion battery pricing is roughly $100 kWh, external.

Some companies that spoke to the BBC suggested sodium ion might reach as low as $30 per kWh but analysts say significantly lower costs could be many years away, external.

Sodium ion batteries are also not as energy dense as lithium ion batteries, meaning they must be slightly bigger and heavier in order to provide the same amount of power.

The technology has been improving, external on this front in recent years but, even so, you're not likely to see sodium ion cells in small, energy hungry devices such as smartphones. Larger energy storage solutions are where most people expect sodium ion to dominate.