Researchers from the University of California, San Diego, have developed all-solid-state sodium batteries that retain performance down to subzero temperatures. These batteries could help replace lithium-based energy devices.
Sodium is an inexpensive, plentiful, and less-destructive alternative to lithium. However, the all-solid-state batteries currently do not work as well at room temperature.
The researchers heated a metastable form of sodium hydridoborate to the point it starts to crystallize, then rapidly cooled it to kinetically stabilize the crystal structure. This technique has not previously been applied to solid electrolytes.
Sodium chemistries
This familiarity could help turn this lab innovation into a real-world product. “It’s not a matter of sodium versus lithium,” said Y. Shirley Meng, Liew Family Professor in Molecular Engineering at the University of Chicago Pritzker School of Molecular Engineering. “We need both. When we think about tomorrow’s energy storage solutions, we should imagine the same gigafactory can produce products based on both lithium and sodium chemistries.”
The research team highlighted that sodium chemistries are attractive, yet sodium solid electrolytes show limited room-temperature ionic conductivity.
Computational and experimental data
The UC San Diego team’s work combines computational and experimental data to evaluate the metastability of sodium hydridoborate. Rapid cooling from the crystallization regime kinetically locks the orthorhombic phase with fast Na+ mobility.
When paired with a chloride-based solid-electrolyte-coated cathode, this metastable phase enables thick, high-areal-loading composite cathodes that retain performance down to subzero temperatures. “Because the underlying principle is kinetic stabilization of a diffusion-favorable anion framework, this approach is transferable to related hydridoborates and other anion-cluster chemistries,” said researchers in a study published on Joule.
Co-first author Sam Oh of the A*STAR Institute of Materials Research and Engineering in Singapore stated that the research helps put sodium on a more equal playing field with lithium for electrochemical performance. “The breakthrough that we have is that we are actually stabilizing a metastable structure that has not been reported,” Oh said.
This metastable structure of sodium hydridoborate has a very high ionic conductivity, at least one order of magnitude higher than the one reported in the literature, and three to four orders of magnitude higher than the precursor itself.
Pairing that metastable phase with an O3-type cathode coated with a chloride-based solid electrolyte can create thick, high-areal-loading cathodes that improve upon previous sodium batteries. “The thicker the cathode is, the theoretical energy density of the battery – the amount of energy being held within a specific area – improves,” Oh said.
The current research advances sodium as a viable alternative for batteries, a vital step to combat the rarity and environmental damage of lithium. It’s one of many steps ahead.
