Researchers in Italy have developed a polymer membrane that allows fluoride ions to move through a battery electrolyte at room temperature, addressing a major obstacle to the development of next-generation fluoride-ion batteries.

The quasi-solid gel membrane, developed by a team at Politecnico di Milano, acts as a fluoride-exchange membrane (FEM), transporting fluoride ions between battery electrodes. The researchers say the approach could help advance an alternative battery chemistry that does not rely on lithium.

Fluoride-ion batteries have attracted interest because fluoride is an abundant element and their conversion-based chemistry offers the potential for high energy density. However, developing electrolytes that can transport fluoride ions efficiently at room temperature has remained a significant challenge.

The study introduces a polymer electrolyte based on a perfluorosulfonic amide ionomer containing tetraalkylammonium fluoride groups.

To improve the movement of fluoride ions, the researchers modified the polymer membrane using a mixture of two solvents, 1,3-dioxolane (DOL) and dimethyl sulfoxide (DMSO). The solvent mixture causes the ionomer to swell, creating conditions that allow fluoride ions to move through the quasi-solid material.

Measurements using electrochemical impedance spectroscopy showed that the membrane achieved ionic conductivity of up to 1.3 millisiemens per centimeter at room temperature.

The team also used molecular dynamics simulations to investigate how the membrane’s structure and solvent composition influence fluoride-ion movement. The simulations indicated that DMSO plays an important role as a polar swelling agent.

The researchers then tested the electrolyte in experimental coin cells using lead and lead fluoride electrodes. The cells demonstrated reversible fluorination and defluorination, the processes through which fluoride ions move into and out of electrode materials during charging and discharging.

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