Fast ion transport in mechanochemically synthesized SnF2 based solid electrolyte, NH4Sn2F5

Abstract

Recent reports on fluoride ion batteries (FIBs) show its promising credentials as suitable alternatives for Li-ion batteries suffering with issues like high cost, limited source, and safety. Unlike to the other alternative battery technologies such as Na, FIBs are in the early stage of progress. Development of solid electrolytes with fast fluoride ion transport is one of the major challenges in FIBs. NH4Sn2F5 belongs to the SnF2 based MSn2F5 (M: K, Rb, and NH4) solid electrolytes exhibiting two-dimensional F ion conduction. The present work reports the structural and transport details of mechanochemically synthesized NH4Sn2F5, where the material is prepared just by milling the initial SnF2 and NH4F powders (2:1) for 10 h without any heat treatment. The formation of the material exhibiting monoclinic structure at room temperature (RT) is confirmed by X-ray diffraction followed by the Rietveld refinement. The temperature dependent conductivity results at different temperature regions are explained with respect to its structural transformations, which is furthermore correlated with the differential scanning calorimetry observation. Later the conductivity values at different temperatures of mechanochemically prepared NH4Sn2F5 are compared with the earlier reported MSn2F5 (M: Na and K) materials synthesized by the same method and the results are explained with respective to their crystal structure. The current study shows that the presence of the larger NH4 cation in MSn2F5 (M: Na, K, NH4) systems causes fast transport of F ions resulting in the highest fluoride ionic (anionic) conductivity value. Like BaSnF4, the solid electrolyte which is often used by many research groups for different FIBs operating at RT, mechanochemically synthesized NH4Sn2F5 has a great potential to be tested as a solid electrolyte in both conversion type and intercalation type FIBs because of its higher RT conductivity value arising from the motion of F ions.