Researchers at the Pacific Northwest National Laboratory have developed a meta-weakly solvating electrolyte that allows for the stable operation of high-voltage sodium-ion batteries.

“Developing alternative battery systems based on earth-abundant elements has thus become increasingly important,” said the researchers in a new study published in the journal Nano Energy.

“Given that sodium is the closest alkali metal to lithium, sharing similar chemistry while being far more available, sodium-ion batteries (SIBs) are naturally positioned as a promising technology for next-generation energy storage.”

In laboratory tests, full cells using this electrolyte retained 80 percent of their initial capacity after 500 cycles. This performance level is higher than that of standard benchmark devices, which typically sustain between 100 and 300 cycles before reaching similar levels of degradation.

“The full cells demonstrate 80% capacity retention after 500 cycles, outperforming both conventional carbonate-based and localized high-concentration electrolytes,” added the study.

The electrochemical testing was conducted at a constant temperature of 30 degrees Celsius using sodium hexafluorophosphate and sodium bis(fluorosulfonyl)imide salts.

Post-cycling analysis to evaluate condition

The researchers also performed post-cycling analysis after 50 cycles, using scanning electron microscopy and energy-dispersive X-ray spectroscopy to evaluate the condition of the electrodes.

The results demonstrated that the new electrolyte formulation improved high-voltage interfacial stability and reduced leakage current when paired with sodium nickel manganese iron oxide cathodes and hard carbon anodes.

Most conventional battery electrolytes are designed to strongly solvate metal ions to assist their movement through the liquid. This process creates a stable ion-solvent shell that can be difficult to break apart when the ion reaches the electrode surface.

When the shell does not detach properly, electrolyte molecules are often pulled into unwanted side reactions at the interface. These reactions form unstable layers and consume the electrolyte, which leads to the gradual degradation of the battery cell materials over time.

The design from the Pacific Northwest National Laboratory utilizes an intermediate solvation structure where sodium ions are less tightly bound to solvent molecules.

“We discovered that replacing conventional non-solvating diluents in LHCEs, such as 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (TTE), with the weakly solvating tris(2,2,2-trifluoroethyl) phosphate (TFP) can address the abovementioned limitations of LHCEs while maintaining an anion-rich environment around sodium ions,” explained the researchers.

Casting a slurry onto aluminium foil

The electrodes were constructed by casting a slurry onto aluminum foil using binders such as polyvinylidene fluoride, sodium carboxymethyl cellulose, and styrene-butadiene rubber, along with conductive carbon additives.

Scientists evaluated the battery’s performance using nuclear magnetic resonance spectroscopy to analyze the specific solvation structures and their behavior at the electrode interface.

These tests showed that the meta-weakly solvating electrolyte allowed for faster sodium desolvation and lower charge-transfer resistance compared to conventional options. Lead author An L. Phan stated that this strategy regulates the sodium solvation structure to facilitate favorable reactions while suppressing unwanted ones.

The result is a reduction in irreversible material loss and improved electrochemical stability during extended cycling.

“The new electrolyte represents a new strategy to regulate Na solvation structure that can facilitate favorable reactions and suppress unwanted ones,” the research’s lead author An L. Phan told ESS News.

“This results in reduced irreversible loss and degradation of cell materials under practical conditions.”