Studies on fluoride ion conductivity of the mechanochemically synthesized -KSbF4 for all-solid-state fluoride-ion batteries

All-solid fluoride ion batteries (FIBs) are considered as the potential alternative to lithium-ion batteries due to their ultra-high volume energy density and high safety. Nevertheless, the major challenge for high-performance FIBs lies in the difficulty of finding a suitable solid-state electrolyte with both high ionic conductivity and easy fabrication. Herein, the air-stable beta-KSbF4 with a high F- ion conductivity of 2.06 x 10-4 S cm-1 at 60 degrees C is firstly synthesized by mechanochemical synthesis combined with low-temperature annealing treatment, and the underlying mechanisms of fast ion conduction in beta-KSbF4 is investigated as well. The analysis of the conductivity spectra based on Jonscher's universal power law shows that the hopping frequency changes by orders of magnitude from 30 degrees C (1.69 x 105 Hz) to 80 degrees C (1.67 x 106 Hz), and the carrier concentration is almost independent of the temperature change. The difference between the two different parameters means that the hopping process of F- ions rather than the concentration of the charge carriers determines the conductivity of beta-KSbF4. Integrating beta-KSbF4 with Ag cathode and Pb + PbF2 anode, a reversible cycling of conversion-type all-solid-state FIBs with a first discharge capacity of 77.4 mAh g-1 at 60 degrees C is achieved and can work stably for 50 cycles without considerable capacity fade. Hence, it is believed that the study on the ionic conduction characteristic of beta-KSbF4 synthesized by the mechanochemical synthesis method and its successful application in all-solid-state FIBs can open a new horizon toward the development of all-solid-state FIBs.