Sodium-Ion Batteries: Safety, Cost and the Race to Build Them in the US

There is ongoing debate about the best and safest sodium-ion battery design. In addition, several US companies active in the field have experienced difficulties in recent years, including Natron, external and Bedrock Materials, external.

China leads sodium-ion battery manufacturing

China currently leads the world in sodium-ion battery manufacturing, meaning US competitors will have to work hard to catch up.

One possibility is that batteries could eventually be manufactured in the United States using 100% American-sourced materials. This would distinguish sodium-ion batteries from lithium-ion batteries, whose materials and supply chains are heavily dominated by China. China is expected to control 39% of global lithium extraction by 2030, external.

For Tan, China helped bring his company to market. Unigrid’s batteries currently use sodium-ion cells manufactured by Unigrid rather than in the United States. “You can switch back and forth and choose which battery to use,” Tan said, explaining that his company may source cells from the US in the future. However, costs may vary.

Are sodium-ion batteries safe?

Unigrid batteries contain chromium-3 in the positive electrode. During operation, electrons flow between the cathode and anode, external, reversing direction when the battery charges and discharges.

Professor Dame Clare Gray of the University of Cambridge, who co-founded battery company Nyobolt, points out that under certain conditions chromium-3 can form chromium-6, a carcinogen, external.

“If you’re going to take in sodium, […] if you’re going to sell safety, don’t start putting in things that will raise red flags,” she told the BBC.

Ivana Hasa of the University of Warwick, who is not affiliated with Unigrid, agrees that chromium-3 can form chromium-6 at very high states of charge. However, she says this risk can be mitigated by controlling the battery’s voltage. “The safety tests reported on cytochemistry are very promising,” she says of Unigrid’s technology.

Tan said concerns about the toxicity of the battery’s materials were “overblown.” He stressed that Unigrid’s batteries had passed certification and safety tests for sale in Europe and were undergoing evaluation for the US market.

According to Tan, third-party tests showed that the battery was unable to form chromium-6. “We tried very hard to produce it and overcharged it as much as possible… We failed to produce chromium-6.”

Battery chemistry affects performance and lifespan

The choice of cathode material and the formulation of the electrolyte separating the cathode and anode are important details that affect the performance and lifetime of a sodium-ion battery. Some manufacturers do not use chromium in the cathode, instead choosing hard carbon or sodium iron pyrophosphate, also known as NFPP.

In general, confidence in the reliability of sodium-ion batteries has increased in recent years. As a result, the technology is beginning to find applications in grid energy storage, says Professor Dame Gray.

How much do sodium-ion batteries cost?

Last December, LFP batteries—a relatively inexpensive type of lithium-ion battery—went on sale at $81 per kilowatt-hour, external (kWh). By comparison, Unigrid’s sodium-ion battery costs approximately $100 per kWh, external.

Some companies that spoke to the BBC suggested sodium-ion batteries could eventually reach around $30 per kWh. Analysts say costs will fall significantly, although that price may still be many years away, external.

Lower energy density limits some applications

Sodium-ion batteries do not have as much energy density as lithium-ion batteries. They therefore need to be slightly larger and heavier to deliver the same amount of power.

Although sodium-ion battery technology has attracted significant attention in recent years, it remains rare to see the batteries installed in small, energy-hungry devices such as smartphones. In large-scale energy storage solutions, however, many observers expect sodium-ion batteries to become increasingly important as the technology improves.