SCI: Fluoride-ion battery research: Charging into the future

As battery needs become more demanding to support sectors such as aviation or long-haul container shipping, alternative ‘beyond-lithium’ battery chemistries are highly sought-after. XiaoZhi Lim reports on research into fluoride-ion alternatives.

Demands for higher energy density in batteries are growing. By one estimate, a battery for a plane needs to pack some 800Wh/kg of energy. While lithium-ion batteries have theoretical energy densities hovering around 500Wh/kg, fluoride ion-batteries could deliver more than 2000Wh/kg [1].

In contrast to lithium, which is highly electropositive, fluorine is the most electronegative element in the periodic table. While lithium ions resist reduction, fluoride ions can withstand oxidation, both allowing for batteries that can operate at high voltages. These two elements are also the smallest and lightest of their groups, leading to energy-dense batteries that are smaller and lighter [2].

‘If you could really make a fluoride battery work, the numbers look great,’ says Simon Jones of California, US-based fluoride battery startup Flion Energy.

However, these numbers remain theory. Fluoride-ion batteries face some technical challenges.

Unlike lithium salts, most simple inorganic fluoride salts are insoluble – a big problem for battery developers looking for electrolytes. Once in solution, fluoride ions are highly reactive due to their strongly nucleophilic nature. But there are other advantages to fluoride-ion batteries, a top one being that fluorine is far more abundant than lithium in the Earth’s crust and the supply chain for fluoride is much larger than that of lithium. Some 9.5m t of fluorspar, or calcium fluoride, were produced in 2024, almost 40 times greater than the production of lithium, of which just 240,000t were produced[3]. ‘Fluoride is quite abundant,’ says Mauro Pasta at the University of Oxford, UK. ‘There is a whole chemical industry in calcium fluoride already.’

High potential

Much of the high energy density promised by fluoride-ion batteries results from the reactivity of fluoride with metals. While lithium-ion batteries function through an intercalation mechanism (see Intercalation batteries box below), most other battery chemistries such as lead-acid or nickel-metal-hydride function through chemical conversion.

‘This nice mechanism of something goes in, something comes out, is only really true for lithium-ion,’ Jones says. ‘Batteries are a lot messier than lithium might lead you to believe.’

In fluoride-ion batteries, metal fluoride cathodes are reduced to their corresponding metals while metallic anodes are oxidised to their corresponding metal fluorides. These redox reactions often come with large differences in free energy, giving rise to high energy potentials. Using iron fluoride as a cathode, for instance, could theoretically offer more than 1400mAhg-1 of specific capacity, while tin and copper fluoride cathodes both offer more than 800mAhg-1. As fluoride ions are stable to oxidation, the cells could also operate at high voltages, another factor that contributes to high energy density in batteries.

The trouble with fluoride-ion batteries, however, lies with the electrolyte. While lithium salts are readily soluble, fluoride salts aren’t. Identifying a suitable medium for conducting fluoride ions was the key for the first working rechargeable fluoride-ion battery, reported by Maximilian Fichtner and Anji Reddy at Karlsruhe Institute of Technology in 2011 [4].

Fichtner and Reddy identified a lanthanum barium fluoride salt as a fluoride conductor to build a solid-state battery. At 150°C, the polycrystalline salt has a fluoride conductivity of about 2x104Scm-1; if it were a single-crystal, the same conductivity could be achieved at 50°C. However, single crystals are impractical for large-scale battery manufacturing. Using the polycrystalline lanthanum barium fluoride salt, the researchers built rechargeable battery cells with copper, bismuth or tin fluoride cathodes and a cerium anode. The cells delivered high capacities of between 131 and 322mAhg-1 while operating at voltages up to 2.5V.

Intercalation batteries

Lithium-ion batteries are unique among batteries because lithium ions simply weave in and out of the crystal lattices of their cathodes and anodes, a process known as intercalation, rather than undergoing chemical reactions. While intercalation-type batteries carry less energy than conversion-type batteries, they are more stable due to the smaller volume changes that occur in the lattices of the cathodes and anodes: just 10-20%, compared with as much as 200% in conversion-type batteries, says Oliver Clemens, of the University of Stuttgart, Germany. Such volume changes can cause the electrodes’ active material to lose contact with the electrolyte.

In Japan, researchers led by Kentaro Yamamoto at Kyoto University, in collaboration with Toyota, reported a cathode that allows intercalation of fluoride ions made of a lanthanum strontium manganese oxyfluoride perovskite (J. Am. Chem. Soc. 2025, DOI: 10.1021/jacs.3c10871). The perovskite’s structure contained enough space within its lattice to fit two fluoride ions, allowing fluoride to leave its lattice during discharge without much change to the structure.

The researchers discovered that the lattice could fit more fluoride ions than it started with, more than doubling the battery’s capacity to almost 200mAhg-1. Upon probing the reaction mechanisms, the researchers found that after fluoride ions have initially filled the lattice’s spaces, oxygen-oxygen bonds formed within the perovskite to create more gaps that accommodated more fluoride ions.

A solid-state coin-cell battery built with this novel perovskite cathode, a lanthanum barium fluoride electrolyte and a lead anode, retained 200mAhg-1 of capacity for 30 cycles without fading, while operating at 140°C.

Solid versus liquid

Solid-state batteries, however, are more limited in applications due to their need for higher temperatures, typically above 60-100°C, to help fluoride ions move faster through the solid electrolytes. Solid-state electrolytes are also heavier than liquid ones, lowering the energy density of a battery. But liquid electrolytes face huge challenges with reactive fluoride ions, which can cause capacity loss and safety problems. In water, for example, fluoride tends to attack water molecules to form a bifluoride, or HF2-, species, Pasta says. Many small organic molecules with acidic hydrogens would also decompose when nucleophilic fluoride ions react with the hydrogens.

Jones and colleagues reported the first liquid electrolyte that conducts fluoride ions in 2018, while working at NASA’s Jet Propulsion Laboratory (JPL), at the California Institute of Technology, US [5]. The team identified neopentyl-substituted quaternary ammonium cations that don’t have hydrogen atoms that fluoride ions could react with. They dissolved the quaternary ammonium fluoride salts in an ether solvent to form a liquid electrolyte with fluoride ions that can move freely at room temperature.

The liquid electrolyte remained stable at operating voltages up to 4V. But when the researchers tested the electrolyte with electrodes of copper, lead or bismuth, and their corresponding fluorides, the metallic electrodes dissolved into the electrolyte, so the battery cells stopped working within ten cycles.

Pasta’s team, meanwhile, has attempted to use concentrated, ca 25-30M, aqueous solutions of caesium fluoride as an electrolyte [6]. These concentrations were so high the researchers dubbed the electrolytes ‘water-in-salt’. They freed up the fluoride ions to become ‘naked’, as fewer water molecules coordinated with each of them. This electrolyte remained stable up to 3V. Tested with copper and lead electrodes, the researchers managed to coax a battery cell with copper and copper fluoride electrodes to cycle up to 30 times.

More recently, Chilin Li and colleagues at the Shanghai Institute of Ceramics, Chinese Academy of Sciences, used a Lewis acid to help better dissolve caesium fluoride in an organic solvent.[7] Lewis acids can stabilise anions, encouraging fluoride ions to go into solution. The best-performing coin-cell battery the researchers prepared with a guanine electrolyte and copper fluoride and lead electrodes delivered an initial capacity of 380mAhg-1, which dropped to 126mAhg-1 after 40 cycles. 

Long-lasting performance

While researchers have prepared laboratory-scale fluoride-ion batteries with high capacity, getting that performance to last has been a big challenge. Figuring out the mechanisms leading to capacity fading is a top research priority, says Oliver Clemens of the University of Stuttgart, Germany. With collaborator Scott Warren at the University of North Carolina, Chapel Hill, US, Clemens and his colleagues discovered that common carbon additives in anodes made with certain metals could potentially cause detrimental side reactions in conversion-type batteries [8].

Almost all fluoride-ion battery electrodes involve a carbon-based additive that can conduct electricity, because the fluoride salts themselves aren’t conductive. With anodes that are made of electropositive metals, the reduction of the carbon additives to a metal carbide becomes favoured over the desired reduction of metal fluoride to metal.

Clemens’ team also stumbled on a mechanism of capacity fading caused by unexpected impurities.[9] By observing how the crystal structure of bismuth fluoride transformed during battery cycling, the researchers noted a small amount bismuth oxide fluoride forming over time. They traced this bismuth oxide fluoride to oxide impurities introduced during the battery cell’s preparation. As the battery charged and discharged, the oxide anions moved too, just like the fluoride anions. ‘The oxide anions are not as fast as the fluoride anions, but they also don’t seem to be completely immobile,’ Clemens says.

Better understanding of the impurities to avoid should help to boost cycling stability.

The interface between electrodes and electrolyte is also a critical area for improvement. Protective layers on electrode surfaces help extend battery life. ‘They’re very thin, and they’re stable, and they prevent further reaction,’ says Will West of NASA’s Jet Propulsion Laboratory (JPL), at the California Institute of Technology, US.

Importantly, once formed the protective layer needs to stop growing: if not it becomes thicker and more difficult for the ions to penetrate the electrodes. For Pasta, an interesting overlap exists between fluoride-ion and lithium-ion batteries, because lithium-ion batteries also rely on fluorinated salts. For example, bis(fluorosulfonyl)imide, is a common additive in lithium-ion batteries as it decomposes to form protective fluoride layers at the electrode-electrolyte interfaces.

‘Understanding the fluorine chemistry within electrolytes through fluoride-ion batteries will empower us to understand and then potentially tackle some of the problems afflicting lithium-ion and lithium-metal batteries as well.’

Non-rechargeable batteries

A rechargeable fluoride-ion battery will likely take many years of work to develop. However, primary or non-rechargeable batteries are much simpler and could be developed quicker.

The lithium-CFx battery is a well-established primary battery deployed in applications including marine, medical, military and space.

Typically, lithium ions travel from a metallic lithium anode into the CFx cathode, which is usually graphite that has been fluorinated with carbon to fluorine in a 1:1 ratio. Theoretically, a lithium-CFx battery could deliver around 1260Whkg-1 if discharged at 4.5V. But practically, the battery operates at only 2.5V and delivers only around 700Whkg-1 of energy.

‘It’s a bit of a puzzle why that is,’ says Will West of NASA’s Jet Propulsion Lab. Many researchers have tried for years to increase the operating voltage closer to that of theoretical. West and his colleagues took a different approach: rather than conduct lithium ions, the researchers instead thought about conducting fluoride ions. ‘We thought, maybe if we change the discharge mechanism entirely, we could get to that higher voltage,’ says West.

In this case, fluoride ions would detach from carbon in the CFx cathode and move the other way, thus reducing CFx to carbon while fluorinating a metallic anode (J. Phys. Chem. C., 2024, DOI: 10.1021/acs.jpcc.4c03412).

For conducting fluoride-ions, the researchers adopted the electrolyte Simon Jones and other JPL researchers reported in 2018. This electrolyte, however, reacts with lithium, so the researchers had to test their idea with a lead or a tin anode, which lowered the theoretical operating voltage and energy, says West.

Regardless, the researchers were able to prove their concept: fluoride ions did exit the CFx cathode, and the batteries carried specific capacities of more than 700 and 400mAhg-1, with lead and tin anodes, respectively.

The researchers hope to unlock higher theoretical energy by developing an electrolyte that would not react with lithium so the CFx cathode can be paired with a lithium anode. Jones and his colleagues are also working to commercialise the fluoride-ion primary batteries for renewable energy applications, particularly in challenging environments where the high-density energy outweighs the need to recharge.