Fluoride battery
Rechargeable battery using fluoride ions as charge carriers.
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Fluoride batteries, also known as fluoride shuttle batteries, are a type of rechargeable battery that moves fluoride ions—the negatively charged form of fluorine—to store and release energy. This technology saw a surge in research interest around the mid-2010s, driven by several appealing features: it is more environmentally friendly, avoids using scarce or geopolitically tricky materials like cobalt and nickel in its electrodes, and offers very high theoretical energy densities. Because the process doesn’t involve plating or stripping metal, using high-capacity metal anodes creates almost no dendrite formation, which improves safety, cycle life, and energy storage. Theoretically, a fluoride battery with a cheap electrode and a liquid electrolyte could achieve energy densities around 800 mAh/g and 4800 Wh/L.
Solid electrolytes
The technology is still in early development, with no commercial products available. The main hurdles include the high reactivity of bare fluoride ions in liquid electrolytes, poor fluoride ion conductivity in solid electrolytes at room temperature, and volume changes in conversion-type electrodes during charging and discharging that put mechanical stress on cell parts, leading to early capacity loss. Despite these issues, fluoride-based technology is considered a candidate for next-generation electrochemical storage.
History
History The concept of shuttling fluoride ions was first proposed in 1974, based on studies of fluoride ion conductivity in calcium fluoride (CaF₂) at temperatures between 400 and 500 °C. Research continued through the 1970s and early 1980s, looking at fluoride conductivity in other inorganic fluorides at high temperatures. A practical attempt came in 1976, when β-PbF₂ doped with potassium fluoride was used as a solid electrolyte in a galvanic cell.
It achieved an open-circuit voltage close to the theoretical value but could not sustain a current under load. Small advances occurred in the 1980s, with a few studies demonstrating working cells using solid fluoride-conducting materials based on lanthanum, lead, or cerium fluoride. These cells still had poor discharge capacity, required high operating temperatures (up to 160 °C), and had short lifetimes compared to commercial batteries.
Lore & Background
The concept of fluoride shuttling was proposed in 1974 during research on fluoride ionic conductivity of CaF₂ at temperatures ranging from 400 to 500 °C. Research continued through the 1970s and early 1980s, with studies on fluoride conductivity of inorganic fluorides at high temperature. In 1976, doping β-PbF₂ with potassium fluoride allowed a galvanic cell to reach open-circuit voltage close to theoretical prediction, but it failed to sustain a current under load. Small advancements in the 1980s produced working cells using solid-state fluoride conductive materials based on lanthanum, lead, or cerium fluoride, but these had unsatisfactory discharge capacity, high working temperature (up to 160 °C), and limited cell life compared to commercially available batteries.
Fluoride batteries drew renewed attention from the mid-2010s, driven by the energy transition and needs for new energy storage devices. Improvements were made in both solid and liquid electrolytes. The chemistry relies on reversible electrochemical fluorination of an electropositive metal at the anode side at the expense of a more noble metal fluoride at the cathode side. Electrodes can be conversion-type, which change crystal structure and allow multiple electron transfers per redox center but suffer volume expansion, or intercalation-type, which insert fluoride ions into a crystal lattice without structural change, offering greater stability but limited to one electron transfer per redox center.
Liquid electrolytes, based on either inorganic fluorides dissolved with anion acceptors or organic tetraalkylammonium fluoride salts, aim to reduce operating temperature and accommodate electrode volume expansion, but face challenges such as low solubility of inorganic fluorides and high nucleophilic reactivity of dissolved fluoride. Solid electrolytes, including tysonite-type rare-earth fluorides and alkaline-earth fluorides like barium-tin fluoride, have been studied but generally achieve insufficient ionic conductivity at room temperature for commercial use, and their stiffness cannot accommodate the volumetric expansion of conversion cathodes. In 2019, a rechargeable fluoride battery was obtained using a BaSnF₄ solid electrolyte covered with an interlayer of LBF, extending the electrochemical stability window.
Reader's Guide
Fluoride battery technology remains in an early stage of development with no commercially available devices. Its significance lies in offering a potential next-generation electrochemical storage technology that avoids scarce and geographically strained mineral resources such as cobalt and nickel, while providing high theoretical energy densities (up to ~800 mAh/g and ~4800 Wh/L). The absence of metal plating and stripping reduces dendrite formation, enhancing safety and cyclability.
However, the technology faces major limitations: high reactivity of naked fluoride in liquid electrolytes, low fluoride ionic conductivity of solid-state electrolytes at room temperature, and volume expansion of conversion-type electrodes that causes mechanical strain and premature capacity fading. Despite these challenges, the fluoride-based approach represents a candidate for future energy storage, with ongoing research into both liquid and solid electrolytes to overcome these barriers. The legacy of fluoride batteries is that of a promising but unproven chemistry, with historical roots in high-temperature solid-state research from the 1970s and renewed interest from the mid-2010s driven by the global energy transition.
Frequently Asked Questions
What are the theoretical energy-density specs of a fluoride battery?
The chemistry promises roughly 800 mAh/g gravimetric capacity and about 4,800 Wh/L volumetric density, figures that dwarf what today's lithium-ion cells can deliver. Those headline numbers are the main reason the mid-2010s research community took such an interest in the platform.
How does a fluoride battery sidestep the weaknesses of lithium-ion?
Because the charge carrier is a small fluoride ion rather than a metal atom, the cell avoids the dendrite-growth problem that limits high-capacity metal anodes. It also removes the dependence on scarce, geopolitically sensitive materials such as cobalt and nickel from its electrode chemistry.
What operating temperature does a fluoride battery require?
The original design called for 400 to 500 °C, which made it far too hot for practical applications. Modern work focuses on solid electrolytes—barium-tin fluoride, for instance, has shown roughly 10⁻⁴ S cm⁻¹ ionic conductivity at room temperature—as a route to ambient-temperature operation.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Fluoride battery (CC BY-SA 4.0).
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