All-solid-state batteries offer a safer and more powerful way to run electric vehicles, power electronics, and store renewable energy from the grid. However, their key ingredient, lithium, is both costly and scarce, and mining it often causes serious environmental harm.
Sodium presents a much cheaper and more abundant alternative, and it is far less damaging to extract. Yet, sodium-based solid-state batteries have long struggled to match lithium's performance at typical temperatures.
"It's not a matter of sodium versus lithium. 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," said Y. Shirley Meng, Liew Family Professor in Molecular Engineering at the UChicago Pritzker School of Molecular Engineering (UChicago PME). "This new research gets us closer to that ultimate goal while advancing basic science along the way."
A new study from Meng's group, published in Joule, takes a major step toward solving that issue. The researchers developed a sodium-based solid-state battery that performs reliably from room temperature to below freezing, setting a new benchmark for the field.
According to first author Sam Oh of the A*STAR Institute of Materials Research and Engineering in Singapore, who conducted the work while visiting Meng's Laboratory for Energy Storage and Conversion, the results bring sodium technology much closer to competing with lithium on electrochemical performance.
The achievement also represents a fundamental advance in materials science.
"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."
Established technique, new field
To create this structure, the researchers heated a metastable form of sodium hydridoborate until it began to crystallize, then cooled it rapidly to lock the structure in place. The method is well known in other areas of materials science but had not previously been used for solid electrolytes, Oh said.
That practical familiarity could make it easier to transition the discovery from laboratory research to industrial production.
"Since this technique is established, we are better able to scale up in future," Oh said. "If you are proposing something new or if there's a need to change or establish processes, then industry will be more reluctant to accept it."
Pairing that metastable phase with a O3-type cathode that has been coated with a chloride-based solid electrolyte can create thick, high-areal-loading cathodes that puts this new design beyond previous sodium batteries. Unlike design strategies with a thin cathode, this thick cathode would pack less of the inactive materials and more cathode "meat."
"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.
"It's still a long journey, but what we have done with this research will help open up this opportunity," Oh said.
