Shielding Fluoride Ion Basicity through Diurea Coordination for Nonaqueous Fluoride Shuttle Batteries.

Abstract

The strong basicity of fluoride ions leads to detrimental nucleophilic attack on organic components in the electrolytes, such as β-hydrogen elimination reactions with organic cations and solvents, converting "naked" F- into corrosive and unstable bifluoride (HF2-) ions. These reactions significantly constrain the choice of suitable solvents and salts to develop electro(chemical) stable fluoride ion electrolytes. In this work, we replaced the triple water ligands typically present in industrial organic fluoride salts with dual 1,3-diphenylurea (DPU) coordination via hydrogen bonding interaction. This modification successfully suppressed the Lewis basicity of fluoride ions, providing long-term chemical stability (over 1000 hours) across a wide range of aprotic solvents, a broadened electrochemical stability window (-2.5 ~ 0.9 V vs. Ag+/Ag) and high ionic conductivity (1.7 mS cm-1) at room temperature. Additionally, the weaker hydrogen bonding in F--DPU coordination, compared to the conventional boron-based anion acceptor (AA) strategy that relies on intensive Lewis acid-base interactions, facilitates faster (de)fluorination kinetics at the electrode. The proposed room temperature fluoride ion batteries sustain improved electrochemical performance by pairing with the Pb-PbF2 anode and BiF3 or Ag cathode.