Ultra-stable potassium storage and hybrid mechanism of perovskite fluoride KFeF ₃ /rGO

Potassium-ion batteries (PIBs) are promising for large-scale energy storage due to the abundant reserves of the element potassium yet few satisfactory cathode materials have been developed due to the limitation of the large ionic radius of the potassium ion. Cubic perovskite fluorides have three-dimensional diffusion channels and a robust structure, which are favorable for ion transfer, but their poor electronic conductivity needs to be compensated. Here, we synthesized cubic KFeF₃ powder by a solvothermal procedure. After the combination with reduced graphene oxide (rGO) and carbon coating, its electronic conductivity is greatly improved. In the optimal sample KFeF₃/rGO-PVA-500, KFeF₃ nano-particles (smaller than 50 nm) distribute on the rGO surface evenly. Owing to the special structure, KFeF₃/rGO-PVA-500 provides an excellent rate performance and cycling stability. In particular, a high capacity retention of 94% is obtained after 1000 cycles at 200 mA g^-1. In addition, a hybrid reaction mechanism combining mainly solid solution and partly conversion processes is revealed by employing in situ and ex situ characterization.