Mechanochemical Synthesis and Fluoride Ion Conductivity Studies in SrSnF4 Polymorphs

Abstract

Investigations on SnF2-based solid electrolytes are gaining significant scientific attention because of their promising applications as solid electrolytes for all-solid-state fluoride ion batteries (FIBs) operating at room temperature. FIBs are potential alternatives for expensive Li-ion batteries with toxic and flammable liquid electrolytes. SrSnF4 belongs to the MSnF4 (M: Pb, Ba, and Sr)-type materials exhibiting a layered structure. Here, we present the structural and transport characteristics of two polymorphs of SrSnF4 using X-ray diffraction and impedance spectroscopy techniques. SrSnF4 crystallizing in a cubic fluorite structure is obtained just by mechanical milling the powder samples of SrF2 and SnF2 taken in a 1:1 ratio for 10 h, whereas annealing the milled powder at 623 K in the N2 atmosphere transforms SrSnF4 from its cubic phase to the stable tetragonal structure. The structural details of both the cubic and tetragonal SrSnF4 have been obtained by performing the Rietveld refinement. The resultant phase change after soft annealing enhances the room-temperature conductivity value from 2.05 × 10–6 S/cm for the cubic phase to 1.16 × 10–5 S/cm for the tetragonal phase. The transport number measurement by the dc polarization technique with the cell of configuration Ag/SrSnF4/Ag reveals that the conductivity is due to the ions. The frequency response of conductivity data is analyzed using the Almond–West formalism to find their hopping frequency and mobile carrier concentration factor at different temperatures. The scaling of the frequency-dependent conductivity spectra shows that the relaxation behavior of the mobile ions is temperature independent.