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The fluoride-ion battery (FIB) is rechargeable battery technology based on shuttle of fluoride ions as ionic charge carrier, with a working principle similar to rocking-chair mechanism in lithium-ion batteries.

This kind of chemistry, conceived in the 1970s came back to attract interest in the mid-2000s because of its environmental friendliness, due to the avoidance of scarce an geographically strained mineral resources in electrodes composition (e.g. cobalt and nickel) and high theoretical energy densities. In addition, since there is no metal plating and stripping, dendrites formation is negligible also if high-capacity metallic anodes are used, with increased safety, cyclability and energy storage capacity. Theoretically, a FIB using a low costs electrodes and a liquid electrolyte can have energy densities as high as 588 Wh kg-1

The fluoride-ion based technology is in early-stage of development and,, there are no commercially available devices. The main issues limiting actual performances are the high reactivity of naked fluoride in liquid electrolytes, low fluoride-ion ionic conductivity of solid-state electrolytes at room temperature and volume expansion of conversion-type electrodes that put mechanical strain on cell components during charging-discharging cycling leading to a premature capacity fading.

History
Batteries based on fluoride-ion shuttling was firstly proposed in 1974 by Baukal et al. while working on fluoride ionic conductivity of CaF2 at temperatures ranging from 400 to 500 °C.

Research continued between 70s and early 80s when other studies about fluoride conductivity of inorganic fluorides at high temperature were carried out, one of the first practical application was made by Kennedy and Miles in 1976 doping β-PbF2 with potassium fluoride. When employed in a galvanic cell as a solid-state electrolyte this material allow reaching open-circuit voltage close to theoretical ones, but failed to sustain a current when a load was applied.

Little advancements were made in the field of FIBs later in the 1980s, with only few studies reported working cells using solid-state fluoride conductive materials based on lanthanum, lead or cerium fluoride with unsatisfactory discharge capacity, if compared to commercially available batteries, high working temperature (up to 160°C) and limited cell life. Fluoride-ion batteris returned to drawing attention in the mid-2000s, driven by energy transition and needs of new energy storage devices. Improvement were made in both solid-state batteries, with the goal of reducing operating temperatures and materials costs, and non-aqueous liquid electrolytes based on ionic-liquids and tetralkylammonium fluorides, that offer larger electrochemical stability widow and good ionic conductivity also at room temperature.

Working principle
The fluoride-ion shuttle batteries chemistry rely on reversible electrochemical fluorination of a electropositive metal (M') at the anode side, at the expenses of a more noble metal fluoride (MFx) at the cathode side.

Discharge process

At Cathode (+)

At Anode (-)

Charge process

At Cathode (-)

At Anode (+)

Conversion-type electrodes
In conversion-type electrodes the redox reaction that occur change the crystal structure of material itself, this process often lead to a big variation of particles volume that can loss contanct with current collector or aggregate and loss active surface area, causing capacity fading. An advantage of converion-type electrodes is the possibility to exploit more than one electron tranasfer per redox center, increasing the specific capacity.

This class include some simple metal and transition metal fluorides, that can exchange two or more electrons per mole, like FeF3, BiF3, CuF2, KBiF3 at cathod ee side or Ca and Mg at anode side.

Intercalation-type
In intercalation-type electrodes fluoride ions can be inserted in a vacancy in the crystal lattice of electrode material, without changing its structure. In this case, the volume variation is greatly reduced making this materials more stable. In contrast, the electron transfer per redox center is usually limited to one, reducing the available specific capacity.

In 2014, Clemens et al. reported electrochemical fluorination of perovskite-type BaFeO2.5 against a CeF3/Ce anode in a solid-state configuration at 150°C. Later in 2017, Nowroozi et al. succesfully proved intercalation of fluoride ion in LaSrMnO4 cathode cycling it against a PbF2/Pb anode with barium doped lanthanum fluoride as solid electrolyte at 150° and 200°C.

Solid-state electrolytes
Due to scarce solubility of inorganic fluorides in non-aqueous solvent solide-state electrolytes was the first solution tested for FIBs, despite intrinsic challenges of such materials. In fact, most fluoride conducting materials achieve sufficient ionic conductivity only at high temperatures (up to 160°C) that limit the possibility of a commercial use. Moreover, the stiffness of these materials doesn't cope well with high volumetric expansion of conversion cathodes.

Tysonite-type rare-earth fluorides
Rare-earth fluorides with tysonite-type structure (RE1-xMxF3-x where RE is a rare-earth among La, Ce, Sm, and M is a second group metal like Ba, Ca or Sr) has attracted interest, because of their wide electrochemical stability windows, up to 4 V vs Li+/Li.

As example, in 2017 Chable et al. synthetized, with a ball milling technique, barium-doped lanthanum fluoride (LBF) reaching an ionic conductivity around 10-5 S cm-1 at room temperature, higher than previous reports but still lower than conventional liquid electrolytes used in commercially available Li-ion batteries. Similar results in terms of ionic conductivity was achieved with cerium fluoride doped with strontium fluoride or calcium-doped samarium fluoride.

Alkaline-earth fluorides
Among alkaline-earth fluorides barium-tin fluoride (BaSnF4) has attracted interest because of its relatively high ionic conductivity at room temperature, in the order of 10-4 S cm-1. Despite the increased ionic conductivity, the low electrochemical stability window of Sn2+ prevent the use of reducing metals as anodes, decreasing maximum cell potential, and consequently, energy density.

In 2019 Mohammad et al. succesfully obtained rechargable FIB with a BaSnF4 solid electrolyte covered with an interlayer of LBF extending the electrochemical stability windows of BaSnF4 without unduly affecting ionic conductivity.

Liquid electrolytes
Liquid electrolytes for FIBs would offer a solution to the problem arising from volumetric expansion of electrodes and at the same time to the reduction of operating temperature, due to intrinsic higher ion mobility, which results in high ion conductivity.

Inorganic fluorides-based electrolytes
Inorganic based liquid electrolytes are made dissolving alkali metal fluorides in a organic aprotic solvents but the low solubility of inorganic fluorides in common battery electrolytes solvent lead to poor ionic conductivity.

To enhance salts solubility, and consequently ionic conductivity, boron-based anion acceptors were used to increase salt solubility in organics, as example, Konishi et al. tested an electrolyte based on caesium fluoride dissolved in tetraglyme with different anion acceptors, including triphenylboroxines and triphenylboranes.

In 2022, Kawauchi et al. succesfully discharged a BiF3 electrode using a solution of Potassium bifluoride dissolved in propylene carbonate with a crown ether as additive to increase salt solubility, achieving a conducibility of ~ 10-3 S cm-1.

Organic fluorides-based electrolytes
Organic based liquid electrolytes were developed dissolving tetralkylammonium salts in proper organic aprotic solvents. The main issues is the high nucleophilic behavior of unsolvated fluoride-ion that react easily with β-hydrogen of alkyl groups via Hofmann elimination mechanism.

To obtain a stable organic-based electrolyte, ammonium salts without β-hydrogen were employed by Davis et al. who tested N,N,N-trimethyl-N-neopentylammonium fluoride dissolved at high concetration in a partially fluorinated ether.