TlSn2F5, a SnF2-based solid electrolyte with high ionic conductivity and electrochemical stability for all-solid-state fluoride ion batteries

Abstract

Fluoride-ion batteries (FIBs) offer better theoretical energy densities and temperature stability, making them suitable alternatives to expensive Li-ion batteries. Major studies on FIBs operating at room temperature focus mainly on MSnF₄ (M: Ba and Pb) solid electrolytes due to their favourable ionic conductivity values. PbSnF₄ is the best fluoride ionic conductor known to date. However, it exhibits poor electrochemical stability. The present work demonstrates the development of TlSn₂F₅ through a single-step mechanical milling method. TlSn₂F₅ exhibits a better ionic conductivity value compared to the earlier reported various solid electrolytes, such as BaSnF₄, KSn₂F₅, and La_0.9Ba_0.1F_2.9, commonly considered for FIBs. Ionic transport number measurement using the dc polarization method indicates that TlSn₂F₅ is an ionic conductor. Furthermore, ¹⁹F NMR spectra measured at various temperatures demonstrate that the rise in conductivity with temperature is attributed to the rapid transport of fluoride ions. The present study indicates that TlSn₂F₅ can be utilized as a potential solid electrolyte for fabricating FIBs.