Fluoride battery

Fluoride batteries (also called fluoride shuttle batteries) are rechargeable battery technology based on the shuttle of fluoride, the anion of fluorine, as ionic charge carriers.

This kind of chemistry is attracted research interest in the mid-2010s because of its environmental friendliness, the avoidance of scarce and 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 fluoride battery using a low cost electrode and a liquid electrolyte can have energy densities as high as ~800 mAh/g and ~4800 Wh/L.

The fluoride based technology is in an 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 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. Despite the aforementioned limitations, the fluoride based technology represents a candidate for the next generation of electrochemical storage technology.

History
Fluoride shuttling was proposed in 1974 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 practical application was made 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 fluoride shuttling later in the 1980s, a 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 batteries started to drawing renewed attention from the mid-2010s, driven by the energy transition and needs of new energy storage devices. Improvements were made in both solid and liquid electrolytes.

Working principle
The chemistry of a fluoride battery 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 transfer 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 BiF3, Bi0.8Ba0.2F2.8, PbF2, FeF3, CuF2, KBiF3 at the cathode side or Ca and Mg at the anode side.

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

Liquid electrolytes
Liquid electrolytes for FBs would offer a solution to the problem arising from the 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 by dissolving alkali metal fluorides in an organic aprotic solvent but the low solubility of inorganic fluorides in common battery electrolytes solvents leads 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 an example, an electrolyte based on cesium fluoride dissolved in tetraglyme with different anion acceptors, including triphenylboroxines and triphenylboranes were discovered.

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

To obtain a stable organic-based electrolyte, ammonium salts without β-hydrogen was employed and tested N,N,N-trimethyl-N-neopentylammonium fluoride dissolved at high concentration in a partially fluorinated ether.

Solid electrolytes
Despite the intrinsic challenges of materials, in fact, most fluoride-conducting materials achieve insufficient ionic conductivity even at high temperatures (up to 160 °C) that limit the possibility of commercial use. Moreover, the stiffness of these materials doesn't cope with the 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 studied, because of their wide electrochemical stability windows, up to 4 V vs Li+/Li.

As an example, in 2017, it was synthesized, with a ball milling technique, barium-doped lanthanum fluoride (LBF), reaching an ionic conductivity of around 10−5 S cm−1 at room temperature but still lower than conventional liquid electrolytes used in commercially available Li-ion batteries. Similar results in terms of ionic conductivity were 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 investigated 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+ prevents the use of reducing metals as anodes, decreasing the maximum cell potential, and consequently, the energy density.

In 2019 researchers obtained rechargeable FB with a BaSnF4 solid electrolyte covered with an interlayer of LBF extending the electrochemical stability windows of BaSnF4.