TECH

Next-generation battery potential unlocked with a novel electrolyte design

A research team has successfully designed a novel electrolyte for fluoride shuttle batteries based on a new concept. The research is published in the journal ACS Applied Energy Materials.

With global demand rapidly increasing for high-energy-density and low-cost energy storage technologies, the search for new systems to replace conventional lithium-ion batteries is accelerating. Fluoride shuttle batteries have garnered significant attention as a promising next-generation candidate. They boast an extremely high theoretical energy density and can be manufactured using inexpensive, abundant materials found in the Earth's crust. The key feature of these batteries is how they operate: They store and release energy by shuttling fluoride ions back and forth between the electrodes.

However, a major issue with this battery system is that the "fluorination reaction" is much harder to trigger than the opposing "defluorination reaction." During fluorination, unwanted side reactions and irreversible processes often occur, causing the battery's performance to decline. Therefore, a crucial challenge has been figuring out how to make this reaction proceed smoothly and what kind of electrolyte design is needed to achieve that.

A bottleneck in fluorination...To promote the fluorination reaction, one approach is to increase the concentration of fluoride ions in the electrolyte. However, stable inorganic fluoride salts generally do not dissolve well in organic solvents, making it difficult to achieve a high enough concentration. Researchers have tried adding specific organic molecules designed to bind to fluoride ions to help them dissolve. These molecules are often expensive, difficult to synthesize and can sometimes trap the fluoride ions too tightly, ironically hindering the very fluorination reaction they were meant to help.

To tackle this problem, the research team focused on a different fluorine-containing inorganic salt: potassium tetrafluoroborate (KBF4). Because KBF4 is chemically stable and is reported to act as a fluoride source in chemical reactions, the team hypothesized that it might effectively regulate the fluorination reaction at the boundary where the electrode meets the electrolyte.

The electrolyte for fluoride shuttle batteries using potassium tetrafluoroborate. Credit:Taketoshi Minato

KBF4 changes the electrolyte...First, the team discovered that by adding both cesium fluoride (CsF) and KBF4 to an organic solvent (tetraglyme), the amount of Cs ions successfully increased dramatically compared with when KBF4 was not used. This suggested that KBF4 boosts the solubility of fluoride salts and fundamentally changes the state of the fluoride ions in the electrolyte.

Next, the team tested the newly prepared electrolyte and confirmed that it possesses high electrochemical stability. Furthermore, using analytical techniques such as cyclic voltammetry and X-ray photoelectron spectroscopy on a bismuth metal electrode, they successfully observed reversible fluorination and defluorination reactions. These results proved that the KBF4-containing electrolyte is highly effective at driving the necessary electrode reactions for fluoride shuttle batteries.

Moreover, in practical charge-discharge measurements, this new electrolyte clearly supported reversible reactions in a bismuth fluoride-composite electrode. Notably, the potential at which the fluorination reaction occurred with this new electrolyte was significantly more negative than in previous systems that used organic additives. This indicates that the KBF4 electrolyte is controlling fluoride ion activity and electrode reactions in a fundamentally different and improved way.

A simpler route to reversibility...These findings demonstrate that KBF4 effectively controls fluoride ion activity within a battery and is a chemically robust, low-cost additive. It is likely that this new electrolyte activates the fluorination reaction by uniquely altering the state of the fluoride ions and the electrode. The team is conducting further research to deepen understanding of exactly how this mechanism works.

Ultimately, this study presents a fresh, simple and scalable approach to designing electrolytes for fluoride shuttle batteries, using materials quite different from previous methods. By proving that a KBF4-based electrolyte enables reversible electrode reactions, this research marks a vital step forward. As scientists continue to improve the electrolyte, optimize electrode structures and stabilize the internal environment of the battery, even greater improvements in capacity, lifespan and practicality can be expected—bringing us closer to a future powered by sustainable, next-generation energy storage.

by: Institute for Molecular Science, Core for Spin Life Sciences, Khon Kaen University and The Graduate University for Advanced Studies