Solid-to-Solid Zn Anode and Interhalogen Iodine Cathodes for High-Voltage Fluoride-Ion Batteries

Fluoride ion batteries (FIBs) operated at room temperature typically suffer from low output voltages (< 0.5 V) due to the lack of reversible, suitable electrode chemistries in liquid electrolytes. Here, we report a high-voltage aqueous FIB system delivering an output voltage of ∼1.3 V by coupling a zinc/zinc hydroxyfluoride (Zn↔ZnOHF) anode with an iodine cathode undergoing reversible I^-↔I₂F^- interhalogen conversion in an NH₄F-based methanol/water hybrid electrolyte. At the anode, strong complexation between NH₄ ⁺ and Zn²⁺ activates a reversible solid-to-solid Zn↔ZnOHF conversion. Meanwhile, methanol effectively suppresses HER and accelerates fluoride-ion desolvation, leading to fast kinetics and high reversibility. As a result, the ZnOHF/Zn anode exhibits remarkable durability for over 1100 h at 1 mA cm^-2 and 2 mAh cm^-2. At the cathode, tetrahexylammonium iodide (THAI) induces the formation of insoluble THAI₂F during the fluorination of I₂ cathode, effectively immobilizing iodine species and suppressing shuttle effects. The proposed fluoride-ion-shuttled Zn/I₂ cells deliver a high specific capacity of 168 mAh g^-1 at 2 A g^-1 and retain 81% capacity after 2500 cycles. This work establishes interhalogen chemistry and electrolyte-regulated solid-state fluorination as viable strategies for designing high-voltage, long-life FIBs.