New electrode design boosts fluoride shuttle batteries

A team of researchers has designed a novel electrolyte for fluoride shuttle batteries based on a new concept.

With global demand rapidly increasing for high-energy-density and low-cost energy storage technologies, the investigation into new systems to replace conventional lithium-ion batteries is accelerating. Fluoride shuttle batteries have garnered attention as a promising next-generation candidate. They feature a 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.

To promote the fluorination reaction, a thought 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.

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

These findings demonstrate that KBF₄ is effectively controlling fluoride ion activity within a battery with a chemically robust and low-cost additive. It is highly likely that this new electrolyte activates the fluorination reaction by uniquely altering the state of the fluoride ions and the electrode. The team are undertaking further detailed research to deepen the 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.

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