Study of behavior of supporting electrolyte ion of fluoride shuttle battery using anomalous X-ray scattering

Fluoride shuttle batteries (FSBs) are superior to lithium-ion batteries (LIBs) in terms of high energy density, safety, etc. An electrolyte consisting of tetraglyme (G4) as a solvent molecule and triphenylboroxine (TPhBX) as an anion acceptor to improve the solubility of cesium fluoride (CsF) salt is a candidate of the electrolytes for FSBs. The low concentration of CsF in the electrolyte makes it difficult to study. Electrical X-Ray total scattering and anomalous X-Ray scattering (AXS) are powerful techniques for studying the local structures of electrolytes. This study shows that AXS measurement with a Cs K-edge can clarify the local structure around very low atomic concentration (0.27 at%) Cs. The first-neighbor distance and the coordination number suggest that Cs${}^{+}$ ions exist in the major Cs(G4)${}_2$${}^{+}$ complex together with the minor Cs(G4)${}^{+}$. The low-Q peak observed in the scattering patterns can be attributed to the structure of alternating [TPhBX]F${}^{-}$ and [Cs(G4)]${}^{+}$ (or [Cs(G4)${}_2$]${}^{+}$). The large sizes of cations ([Cs(G4)]${}^{+}$ and [Cs(G4)${}_2$]${}^{+}$) and anion [TPhBX]F${}^{-}$ lead to a long correlation distance. This work presents the picture that the F${}^{-}$ ions hop from TPhBX to [Cs(G4)]${}^{+}$ or [Cs(G4)${}_2$]${}^{+}$ toward the cathode and anode during charge and discharge, respectively. (© 2022 The Authors. Advanced Energy and Sustainability Research published by Wiley‐VCH GmbH)