‘It’s not a matter of sodium versus lithium, we need both’

As lithium-ion batteries continue to improve in terms of both performance and cost, it is becoming increasingly difficult for alternative technologies to challenge the incumbent. However, interest in solid-state batteries, which promise better energy density and safety, has significantly increased in recent times.

Similar to lithium-ion batteries, solid-state batteries store energy and release it to power devices. But instead of the liquid or polymer gel electrolytes used in lithium-ion cells, solid-state batteries rely on a solid electrolyte.

Building on this principle, researchers at the University of Chicago have developed a new sodium-based solid electrolyte that can reportedly maintain performance even at subzero temperatures.

“It’s not a matter of sodium versus lithium. We need both,” said Y. Shirley Meng, Liew Family Professor in Molecular Engineering at the UChicago Pritzker School of Molecular Engineering. “When we think about tomorrow’s energy storage solutions, we should imagine the same gigafactory producing products based on both lithium and sodium chemistries.”

The team identified a particular metastable structure of sodium hydridoborate with very high ionic conductivity. By rapidly cooling it, they were able to stabilize its crystal structure. This solid electrolyte was then paired with an ozone-based cathode coated with a chloride electrolyte to create high-areal-loading cathodes.

Next, the thick cathode layer was combined with a high-capacity alloy anode. This pairing, together with the rapid cooling of sodium hydridoborate, effectively “locked in” the material’s crystal structure while allowing fast sodium-ion (Na⁺) mobility.

“High-throughput molecular dynamics simulations reveal that the propensity for anion motion significantly enhances the population of highly mobile Na⁺ without affecting the activation energy,” the researchers explained.

“This metastable structure of sodium hydridoborate exhibits ionic conductivity at least ten times higher than previously reported—and three to four orders of magnitude higher than its precursor,” said co-author Sam Oh.

The team believes that using thicker cathodes could boost the energy density of solid-state batteries. “It’s still a long journey, but this research opens up new possibilities for the technology,” Oh said.

The research work can be found in “Metastable sodium closo-hydridoborates for all-solid-state batteries with thick cathodes,” published in Joule.