US Startups Bet Sodium Ion Can Rebuild American Battery Making

San Diego-based Unigrid has shipped 100 home batteries to European customers sweltering through this summer's heatwaves and expects to ship 1,000 more before the end of the year. The surge in demand came from homeowners seeking to store cheap off-peak grid power and run air conditioners during the hottest hours.

"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 nine kilowatt hour battery is a sodium ion device, not lithium ion. That distinction sits at the center of a broader bet by US startups: that sodium ion batteries could be much cheaper to build and operate than lithium ion, less prone to catching fire, and able to work across a wider range of temperatures, down to minus 40C.

The commercial logic is straightforward. Lithium ion batteries depend on a materials supply chain dominated by China, which also leads the world in sodium ion manufacturing. Sodium ion offers the possibility of batteries made in the US with 100% US-sourced materials. China's head start is not permanent, insists Cameron Wiles, president and co-founder of Peak Energy, a California- and Colorado-based firm that announced a partnership with General Motors in June. "China does not have an insurmountable lead in sodium ion batteries," he says.

Peak Energy and GM aim to develop a US supply chain for sodium ion batteries to enable large-scale grid energy storage. The company hopes to help GM launch a US-based production facility in Michigan "by the end of 2028," Wiles says. Like its peers, Peak Energy currently relies on China-sourced cells for its first installations.

That dependence on Chinese cells is a common thread. Unigrid's battery uses sodium ion cells made in China. "We can switch back and forth and choose what battery we want to work with," says Tan, noting his firm might source cells from the US in future, though at a different cost. Emerald Battery Labs, based in Seattle, is also developing its product — 12 volt auxiliary vehicle batteries now supplied to unnamed US commercial fleets — using China-sourced cells. "We took a look at the market and said 'Hey, no-one's trying to attack this space, let's have Emerald Battery Labs attack it,'" says co-founder and CEO David Bell. The firm's next goal is developing new anodes for US-produced sodium ion cells.

The chromium question

Unigrid's battery contains chromium-3 in its cathode. Prof Dame Clare Grey of the University of Cambridge, co-founder of battery firm Nyobolt, points out that under certain conditions chromium-3 may form chromium-6, a toxic, cancer-causing material. "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 says.

Ivana Hasa at the University of Warwick, who has no connection to Unigrid, agrees chromium-3 can form chromium-6 at very high states of charge, though she says voltage control within the battery can mitigate the risk. "Safety tests reported on the cell chemistry are very promising," she says of Unigrid's technology.

Tan calls toxicity concerns "overblown." He says Unigrid's battery has passed certification and safety tests for sale in Europe and is undergoing US evaluations. Third-party tests could not produce chromium-6, he adds. "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."

Cathode chemistry is a key design battleground. Some manufacturers avoid chromium entirely, using hard carbon or sodium iron phosphate pyrophosphate (NFPP) instead. Grey notes that confidence in sodium ion reliability has increased in recent years, and the technology is now entering grid storage applications.

The cost gap

The economics remain sodium ion's biggest hurdle. Last December, LFP batteries — a relatively cheap form of lithium ion — traded at $81 per kilowatt hour. Unigrid's sodium ion battery is priced at roughly $100 per kWh. Some companies that spoke to the BBC suggested sodium ion might eventually reach $30 per kWh, but analysts say significantly lower costs could be many years away.

Sodium ion batteries are also less energy dense than lithium ion, meaning they must be bigger and heavier to deliver the same power. That rules them out of smartphones and small, energy-hungry devices. Large-scale storage is where most observers expect the technology to dominate. "We think sodium ion […] is going to make up 80% of the market," says Bell of Emerald Battery Labs.

One demand driver is the electricity consumption boom from data centres, which is pushing up greenhouse gas emissions in the US and beyond, including the UK. Drew Buckley, chief executive of Oregon-based ESS Tech, calls this an "opportunity," noting that sodium ion batteries could distribute renewable energy to data centres and should be cheaper to run than lithium ion because they require less cooling. ESS Tech is working with Alsym, another US firm planning to ship "hundreds" of megawatt hours' worth of US-made sodium ion batteries by 2027.

Hard road ahead

Hasa cautions that mass production of sodium ion batteries will be "challenging" and that the technology, often touted as much safer than lithium ion, still carries fire risk. More than one US company in the space has already floundered in recent years. But she argues that success in this field could open new strategic options: avoiding wastage of renewable electricity and keeping energy costs low. "I'm a strong believer in the technology," says Hasa.