Complexation of F- by Li+ and Mg2+ Ions as Inorganic Anion Acceptors in Lactone-Based Li+/F- and Mg2+/F- Hybrid Electrolytes for Fluoride Shuttle Batteries

The development of high-quality fluoride-ion transporting electrolytes is a crucial demand for fluoride shuttle batteries (FSBs). However, the uncontrolled chemical and electrochemical activities of fluoride ions narrow the available potential window, hindering the development of high-voltage FSB cells. We present a method for upgrading recently developed lactone-based liquid fluoride electrolytes by complexation of F- with Li+ and Mg2+ ions. In the resultant Li+/F- and Mg2+/F- hybrid electrolytes, Li2F+ and MgF+ were the most probable soluble complexes, and the effective fluoride concentrations could reach similar to 0.15 M along with excess Li+(Mg2+) ions. Unique interactions between F- and Li+(Mg2+) were observed using F-19 nuclear magnetic resonance spectroscopy. Li+(Mg2+) ions thus served as inorganic anion acceptors with ultimate redox stabilities to expand the negative potential window of the electrolytes to near -3 V vs SHE. The proposed complex formation was also supported by a conductometric titration method. We demonstrated the superior and versatile electrochemical performances of the Li+/F- hybrid electrolyte, which enabled reversible charge/discharge reactions of various metal electrodes and composite electrodes in a wide range of redox series. Further, the Li+/F- hybrid electrolyte opened valid new reaction paths for aluminum, making it a promising negative electrode in high-voltage FSB cells.