Reducing Voltage Hysteresis of FeF₂ via Oxygen Doping and Nano-Disordering in Sulfide All-Solid-State Batteries

Conversion-type metal fluorides are promising cathode candidates for high-energy-density post-lithium batteries. Unfortunately, their practical implementation has been severely impeded by pronounced voltage hysteresis (VH). Herein, we report a gas-phase treatment strategy to realize oxygen doping and nano-disordering of FeF₂, which reduces the VH of FeF₂ to a record low level of 153 mV at 80°C when integrated into sulfide all-solid-state batteries. Specifically, crystalline FeF₂ is doped with oxygen and refined to nanocrystals dispersed in a disordered FeOF matrix, which narrows the band gap of FeF₂ from 1.84 to 1.36 eV, decreases the ionic migration energy barrier from 2.2 to 1.2 eV, enhances its electronic conductivity from 4.1 × 10^-6 mS/cm to 0.48 mS/cm, ionic conductivity from 1.1 × 10^-6 mS/cm to 4.8 × 10^-5 mS/cm. The dramatically increased electronic and ionic conductivity boosts the charge transport kinetics and reduces the charge transfer barrier, thus suppressing the VH. Notably, the oxygen doping strategy is not restricted to FeF₂, but is valid for a broad class of metal fluorides. These results provide a versatile solution to the long-standing VH bottleneck in metal fluorides and are expected to accelerate the industrial adoption of metal fluoride cathodes to enable high-energy-density lithium batteries.