Study of Behavior of Supporting Electrolyte Ion of Fluoride Shuttle Battery Using Anomalous X‐Ray Scattering

Abstract

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.

Keywords

• electrolytes
• fluoride shuttle batteries
• pair distribution function analysis
• rechargeable batteries
• X-Ray anomalous scattering