In Situ Observation of Fluoride Shuttle Battery Reactions with Dissolution-Deposition Mechanisms by Raman Microscopy

Abstract

Fluoride shuttle batteries (FSBs) are expected to have high energy densities. Evolution of FSB reactions, defluorination and fluorination of a nearly two-dimensional cluster of orthorhombic BiF3 microparticles partly embedded in a gold plating film (o-BiF3/gold) was studied by in-situ Raman microscopy. Solutions of fluorobis(2,4,6-trimethylphenyl)borane (FBTMPhB) and CsF (0.45 M and about 0.5 M (saturation)) in tetraglyme were used as electrolytes. During defluorination in the 0.45 M CsF solution, metallic bismuth appeared not only at positions of the cluster but also at positions of the gold plating, accompanying changes in morphology of the cluster, indicating that a dissolution-deposition mechanism (BiF3 → Bi3+ + 3F−, Bi3+ + 3e− →Bi) was operative in defluorination. Deposition of metallic bismuth preferentially occurred in the vicinity of the cluster, resulting in extension of protruding areas on the surface. BiF3 with a hexagonal structure appeared during fluorination together with change in the morphology of the cluster. With 0.5 M CsF solution, changes in morphology of the cluster during defluorination and fluorination were small, suggesting that the contribution of the dissolution-deposition mechanism was small. The observed changes in distribution and morphology of active materials will be useful information for designing better electrolytes and electrodes in future batteries.