Advancing Fluoride-Ion Batteries with a Pb-PbF₂ Counter Electrode and a Diluted Liquid Electrolyte

Abstract

Fluoride ion batteries (FIB) are a promising post lithium-ion technology thanks to their high theoretical energy densities and Earth-abundant materials. However, the flooded cells commonly used to test liquid electrolyte FIBs severely affect the overall performance and impede comparability across different studies, hindering FIB progress. Here, we report a reliable Pb-PbF 2 counter electrode that enables the use of two-electrode coin cells. To test this setup, we first introduce a liquid electrolyte that combines the advantages of a highly concentrated electrolyte (tetramethylammonium fluoride in methanol) while addressing its transport and high-cost shortcomings by introducing a diluent (propionitrile). We then demonstrate the viability of this system by reporting a BiF 3 −Pb-PbF 2 cell with the highest capacity retention to date.

Anhydrous N,N-Dimethyl-N,N-Dineopentylammonium Fluoride Electrolyte for Fluoride Ion Batteries

“…Most FIBs use solid electrolytes that require high temperatures to operate efficiently. − One way to reduce the operating temperature of the battery is to switch to liquid electrolytes, which retain high ionic conductivity down to room temperature. − However, successfully implementing liquid electrolytes requires overcoming the challenges of solubility and stability. While liquid electrolytes such as ammonium bifluoride, metal bifluoride, can be readily prepared in usefully high concentrations, the presence of protonated fluorides lowers the electrochemical window below the threshold needed for good battery performance. − On the other hand, quaternary ammonium fluoride salts such as tetramethylammoniumin fluoride (TMAF) can be expected to have wide voltage windows due to the absence of protonated fluorides.…”

Fluoride-Based Deep Eutectic Solvents with Amide Dual-Hydrogen-Bond Donors

“…Fluorination endows compounds with unique properties because of the unique characteristics of fluorine, such as its small atomic radius and highest electronegativity, thereby playing important roles in various fields, including material science, energy storage, and medical science. , Developing F – -containing electrolytes is crucial for electrochemical and chemical fluorination. As such, their applications to the electrochemical synthesis of fluoroorganic compounds and fluoride-ion batteries (FIBs) have been investigated. − …”

“…The solubility of organic fluoride salts is higher than that of inorganic fluoride salts; thus, it was enhanced by modifying the cation structures . The solvents used to dissolve fluoride salts are also important for achieving a high F – concentration, and bis­(2,2,2-trifluoroethyl) ether (BTFE) provides a value of 2.2 mol dm –3 for N , N , N -trimethyl- N -neopentylammonium fluoride (Np 1 F). , Meanwhile, protic solvents endow fluoride salts with high solubility; however, their low electrochemical stability and suppressed F – reactivities result in issues for (electro)­chemical fluorination. − In recent studies, electrochemical stability was improved by dissolving high concentrations of fluoride salts. , …”